A simplified version is shown below.

{
 "DATASIZE": {
    "x": 4096,
    "y": 4004
  },
  "DETSIZE": {
    "x": 4352,
    "y": 4096
  },
  "NUM_AMPS": 16,
  "SEG_DATASIZE": {
    "x": 512,
    "y": 2002
  },
  "SEG_SIZE": {
    "x": 544,
    "y": 2048
  },
  "BOUNDARY": [
    [
      {},...,{}
    ],
    [
      {},...{}
    ]
  ],
  "BOUNDARY_OVERSCAN": [
    [
      {},...{}
    ],
    [
      {},...{}
    ]
  ]
}
Inside BOUNDARY or BOUNDARY_OVERSCAN field each segment is being described as:

      {
        "width": 544,
        "height": 2048,
        "x": 10,
        "y": 2094,
        "index": 15,
        "reverse_slice": {
          "x": false,
          "y": true
        }
      }
Keywords x, y, width and height are describing a region as a rectangular box (similar to ds9 region).
index is the index of segment/extension in the multi-extension FITS file.
reverse_slice is describing the orientation (whether the slicing in data/matrix should be reversed or not for displaying) in X and Y direction, which is used when converting multi-extension to single-extension FITS. I think it's not very helpful in the front end since the image displayed is already single-extension.

Front end can get information of a particular segment by indices (assume 8×2 segments). For example,

json_var["BOUNDARY_OVERSCAN"][0][7]
Here is the complete version.

{
  "BOUNDARY": [
    [
      {
        "height": 2002,
        "index": 15,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 512,
        "x": 0,
        "y": 2002
      },
      {
        "height": 2002,
        "index": 14,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 512,
        "x": 512,
        "y": 2002
      },
      {
        "height": 2002,
        "index": 13,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 512,
        "x": 1024,
        "y": 2002
      },
      {
        "height": 2002,
        "index": 12,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 512,
        "x": 1536,
        "y": 2002
      },
      {
        "height": 2002,
        "index": 11,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 512,
        "x": 2048,
        "y": 2002
      },
      {
        "height": 2002,
        "index": 10,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 512,
        "x": 2560,
        "y": 2002
      },
      {
        "height": 2002,
        "index": 9,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 512,
        "x": 3072,
        "y": 2002
      },
      {
        "height": 2002,
        "index": 8,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 512,
        "x": 3584,
        "y": 2002
      }
    ],
    [
      {
        "height": 2002,
        "index": 0,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 512,
        "x": 0,
        "y": 0
      },
      {
        "height": 2002,
        "index": 1,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 512,
        "x": 512,
        "y": 0
      },
      {
        "height": 2002,
        "index": 2,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 512,
        "x": 1024,
        "y": 0
      },
      {
        "height": 2002,
        "index": 3,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 512,
        "x": 1536,
        "y": 0
      },
      {
        "height": 2002,
        "index": 4,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 512,
        "x": 2048,
        "y": 0
      },
      {
        "height": 2002,
        "index": 5,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 512,
        "x": 2560,
        "y": 0
      },
      {
        "height": 2002,
        "index": 6,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 512,
        "x": 3072,
        "y": 0
      },
      {
        "height": 2002,
        "index": 7,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 512,
        "x": 3584,
        "y": 0
      }
    ]
  ],
  "BOUNDARY_OVERSCAN": [
    [
      {
        "height": 2048,
        "index": 15,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 544,
        "x": 10,
        "y": 2094
      },
      {
        "height": 2048,
        "index": 14,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 544,
        "x": 554,
        "y": 2094
      },
      {
        "height": 2048,
        "index": 13,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 544,
        "x": 1098,
        "y": 2094
      },
      {
        "height": 2048,
        "index": 12,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 544,
        "x": 1642,
        "y": 2094
      },
      {
        "height": 2048,
        "index": 11,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 544,
        "x": 2186,
        "y": 2094
      },
      {
        "height": 2048,
        "index": 10,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 544,
        "x": 2730,
        "y": 2094
      },
      {
        "height": 2048,
        "index": 9,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 544,
        "x": 3274,
        "y": 2094
      },
      {
        "height": 2048,
        "index": 8,
        "reverse_slice": {
          "x": false,
          "y": true
        },
        "width": 544,
        "x": 3818,
        "y": 2094
      }
    ],
    [
      {
        "height": 2048,
        "index": 0,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 544,
        "x": 22,
        "y": 0
      },
      {
        "height": 2048,
        "index": 1,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 544,
        "x": 566,
        "y": 0
      },
      {
        "height": 2048,
        "index": 2,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 544,
        "x": 1110,
        "y": 0
      },
      {
        "height": 2048,
        "index": 3,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 544,
        "x": 1654,
        "y": 0
      },
      {
        "height": 2048,
        "index": 4,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 544,
        "x": 2198,
        "y": 0
      },
      {
        "height": 2048,
        "index": 5,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 544,
        "x": 2742,
        "y": 0
      },
      {
        "height": 2048,
        "index": 6,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 544,
        "x": 3286,
        "y": 0
      },
      {
        "height": 2048,
        "index": 7,
        "reverse_slice": {
          "x": true,
          "y": false
        },
        "width": 544,
        "x": 3830,
        "y": 0
      }
    ]
  ],
  "DATASIZE": {
    "x": 4096,
    "y": 4004
  },
  "DETSIZE": {
    "x": 4352,
    "y": 4096
  },
  "NUM_AMPS": 16,
  "SEG_DATASIZE": {
    "x": 512,
    "y": 2002
  },
  "SEG_SIZE": {
    "x": 544,
    "y": 2048
  }
}