CustomFieldTypes:

B = Boolean (Checkbox)
S = Selection (Dropdown)
T = Text
D = Date
N = Number
M = Money (Currency)

{
    "customFieldId": 157,
    "customFieldParentId": null,
    "customFieldName": "Education level",
    "customFieldGroupId": 52,
    "customFieldGroupName": "Education",
    "customFieldGroupType": "Contacts",
    "customFieldTypeId": "S",
    "isEditable": true,
    "isExportable": false,
    "maxTextboxCharacters": null,
    "availableValues": [
      {
        "id": 1,
        "name": "High School diploma",
        "parentValueId": null
      },
      {
        "id": 2,
        "name": "College degree",
        "parentValueId": null
      },
      {
        "id": 3,
        "name": "Postgraduate degree",
        "parentValueId": null
      },
      {
        "id": 4,
        "name": "Doctorate",
        "parentValueId": null
      }
    ]
}

Custom Field

Property	Type	Description
`customFieldId`	int	Unique identifier for a Custom Field in this context
`customFieldParentId`	int	Identifies a Selection’s parent (Only applies to hierarchical Selections)
`customFieldName`	string	The name for the Custom Field, no longer than 255 characters
`customFieldGroupId`	int	Unique identifier for the Custom Field Group to which this Custom Field belongs
`customFieldGroupName`	string	The name of the Custom Field Group to which this Custom Field belongs, no longer than 60 characters
`customFieldGroupType`	string	The type of Custom Field Group (must be Contacts, Contributions, or Disbursements)
`customFieldTypeId`	string	A one character identifier of the Custom Field type
`isEditable`	bool	Indicates whether or not the value assigned to the Custom Field can be changed
`isExportable`	bool	Indicates whether or not the Custom Field may be included in an Export Job
`maxTextboxCharacters`	int	The maximum number of characters allowed in the text field. (Only applies to Text or Number fields)
`availableValues`	array	An array of available values. (Only applies to Selections)

Available Value

Property	Type	Description
`id`	int	The value of the Selection option
`name`	string	The text of the Selection option, no longer than 100 characters
`parentValueId`	int	The id of the the Selection option in the Custom Field’s parent Selection (Only applies to hierarchical Selections)