Subscription record include information about group and current user rights and reading state for it. In full form it contains also <group record> with participating users records:

{
  "subscription": {
    {
      "id": 1958,
      "group_id": 252,
      "user_id": 1508,
      "role": "rw",
      "mute_until": null,
      "draft": null,
      "tags": null,
      "last_read_message_id": null,
      "created_at": "2018-10-16T18:03:27Z",
      "group": {
        "id": 252,
        "name": null,
        "is_deleted": false,
        "is_space": false,
        "created_at": "2018-10-16 18:03:27 UTC",
        "updated_at": "2018-10-16 18:03:27 UTC",
        "last_message_id": null,
        "last_serial": null,
        "strid": "p2p:1508:1509",
        "type": "private_chat",
        "participants": [
          {
            "id": 1958,
            "group_id": 252,
            "user_id": 1508,
            "role": "rw",
            "mute_until": null,
            "draft": null,
            "last_read_message_id": null,
            "created_at": "2018-10-16T18:03:27Z",
            "user": {
              "id": 1508,
              "nick": "test_user_2",
              "is_online":true
            }
          },
          {
            "id": 1959,
            "group_id": 252,
            "user_id": 1509,
            "role": "rw",
            "mute_until": null,
            "draft": null,
            "last_read_message_id": null,
            "created_at": "2018-10-16T18:03:27Z",
            "user": {
              "id": 1509,
              "nick": "test_user_3",
              "is_online":false
            }
          }
        ]
      },
      "user": {
        "id": 1508,
        "nick": "test_user_2"
      }
    }
  ]
}

in short form. group object will not be included. It can be requireset separately.

Note that tags field if exists, contain array of strings. This field is shown to the subscription owner only.