Documentation missing for fields in the request for custom fields

Unfortunately, a long search did not help me understand the purpose of all fields.

   "id": 12451
   "key": "title",
   "name": "Title",
   "order_nr": 17,
   "picklist_data": null,
   "field_type": "varchar",
   "add_time": "2018-10-09 04:44:11",
   "update_time": "2018-10-09 04:44:06",
   "active_flag": true
   "edit_flag": false,
   "index_visible_flag": true
   "details_visible_flag": false,
   "add_visible_flag": false,
   "important_flag": false,
   "bulk_edit_allowed": true
   "searchable_flag": false,
   "filtering_allowed": true
   "sortable_flag": true
   "use_field": "id",
   "link": "/ deal /",
   "mandatory_flag": true

I would also like to receive some information on how to distinguish custom fields (the key is 40-character custom fields) from the standard ones.

You can find details about the custom fields in the /<entity>Fields endpoints. For instance