Activity Logs documentation does not contain Entity Type "billing_group"

I’m processing the Activity Log event stream, and have run into an undocumented entity type called billing_group. It is not listed in the Activity Logs Types.

{
  "id": "123456",
  "timestamp": 1234567,
  "actor": {
    "type": "user",
    "id": "9876543",
    "name": "Example User",
    "email": "user@example.com"
  },
  "action": {
    "type": "license_group_admins_add",
    "details": {
      "new_admin_emails": [
        "admin@example.com"
      ]
    }
  },
  "entity": {
    "type": "billing_group",
    "id": "12345",
    "name": "Fictitious Client Name"
  },
  "context": {
    "team_id": null,
    "org_id": "123456789",
    "ip_address": "127.0.0.1",
    "is_figma_support_team_action": false,
    "client_name": null
  }
}