This is one of many protocol behaviors described in ActivityPub.

Identifier

urn:uuid:cc3f730a-37a9-4af9-948f-7c8a0b7f6c41

Content

actor objects must have an inbox property

Origin

prefix 4.1 Actor objects

Actor objects MUST have, in addition to the properties mandated by 3.1 Object Identifiers, the following properties: inbox A reference to an [ActivityStreams] OrderedCollection comprised of all the messages received by the actor; see 5.2 Inbox.
 
suffix outbox An [ActivityStreams] OrderedCollection comprised of all the messages produced by the actor; see 5.1 Outbox.

https://www.w3.org/TR/activitypub/

JSON

{
  "id": "urn:uuid:cc3f730a-37a9-4af9-948f-7c8a0b7f6c41",
  "type": "Behavior",
  "uuid": "cc3f730a-37a9-4af9-948f-7c8a0b7f6c41",
  "content": "actor objects must have an inbox property",
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#actor-objects",
      "branch": [
        4,
        1
      ]
    },
    "selector": {
      "type": "TextQuoteSelector",
      "exact": "Actor objects MUST have, in addition to the properties mandated by 3.1 Object Identifiers, the following properties:\ninbox\n  A reference to an [ActivityStreams] OrderedCollection comprised of all the messages received by the actor; see 5.2 Inbox. \n",
      "prefix": "4.1 Actor objects\n",
      "suffix": "outbox\n  An [ActivityStreams] OrderedCollection comprised of all the messages produced by the actor; see 5.1 Outbox.\n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}