Parameter	Mandatory	Description	Examples	More Info
API	Yes	The API area	clients.json
General	Yes	Parameters described in overview	user=user&password=user	Structure of an API call
Model Structure	Yes	Name of model required	modelstructure=clientdetails	Using data models
View	No	Stored filter applied		Using views
Sort	No	Change the view's default sort order	sort=CLIENT	Sorting
Filters	No	Filter by a key field		Filters
Company	Yes	The company number	company=1
Identifier	Yes	Client ID	client=1/P0909

Example request:

http://<SERVER>/jsonapi/clients.json?company=1&version=4&user=user&password=user&company=1&style=modeldata&modelstructure=clientdetails&client=1/P0909

Example response:

{
  "responsecode": 1,
  "responsestatus": "OK",
  "errorcode": 0,
  "errormessage": "",
  "totalrows": 1,
  "totalpages": 1,
  "responsemessage": "",
  "pagerows": 0,
  "pagenumber": 1,
  "success": true,
  "data": [
    {
      "clientUuid": "493686EA86774194831FFE438B031917",
      "clientTimeStampModified": 1623666082,
      "clientRecordVersionNumber": 103,
      "clientCustomField196": "a longer item to select",
      "clientCustomField260": "",
      "clientCustomField296": "1",
      "clientCustomField315": false,
      "clientCustomField323": "",
      "clientCustomField524": "0000-00-00",
      "clientCustomField561": false,
      "clientCustomField562": "",
      "clientCustomField564": false,
      "clientCustomField565": false,
      "clientCustomField566": false,
      "clientCustomField577": "",
      "clientCustomField578": "00:00:00",
      "clientNotes": "",
      "clientCode": "1/P0909",
      "clientName": "New Company & co",
      "clientStreet": "Hollins Park, Hollins Lane",
      "clientAddress1": "",
      "clientAddress2": "Warrington",
      "clientCounty": "http://www.synergist.co.uk",
      "clientPostcode": "WA2 8WA",
      "clientCountry": "",
      "clientCountryCode": "UK",
      "clientCountryName": "United Kingdom",
      "clientPhone": "",
      "clientFax": "",
      "clientHandlerUserID": 568,
      "clientHandlerFullName": "Aaron Dyson",
      "clientWebURL": "www.synergist.co.uk",
      "clientEmail": "",
      "clientISDN": "",
      "clientInternalClient": false,
      "clientInactive": false,
      "clientOPSourceClient": "",
      "clientType": "",
      "clientEmployees": "",
      "clientOpStatus": "",
      "clientMarketSector": "Animation",
      "clientDefaultSpec": "",
      "clientLeadRating": "",
      "clientOnHold": false,
      "clientTimeZoneCode": "UTCA",
      "clientTimezoneDescription": "",
      "clientCompanyNumber": 1,
      "clientCategory": 3,
      "clientCategoryName": "Client",
      "clientPricesReviewNextDate": "0000-00-00",
      "clientPricesReviewNotes": ""
    }
  ]
}

Note: needs 12.2 for custom fields