This is one of many protocol behaviors described in ActivityPub.

Identifier

urn:uuid:7eb4f1aa-5779-4935-9840-f2909a5c6d70

Content

Servers MUST limit the number of layers of indirections through collections which will be performed, which MAY be one.

Origin

prefix If a recipient is a Collection or OrderedCollection, then the server MUST dereference the collection (with the user's credentials) and discover inboxes for each item in the collection.

Servers MUST limit the number of layers of indirections through collections which will be performed, which MAY be one.
 
suffix Servers MUST de-duplicate the final recipient list.

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

JSON

{
  "id": "urn:uuid:7eb4f1aa-5779-4935-9840-f2909a5c6d70",
  "uuid": "7eb4f1aa-5779-4935-9840-f2909a5c6d70",
  "type": "Behavior",
  "content": "Servers MUST limit the number of layers of indirections through collections which will be performed, which MAY be one.\n",
  "context": [
    "https://www.w3.org/TR/activitypub/",
    {
      "name": "7. Server to Server Interactions",
      "id": "https://www.w3.org/TR/activitypub/#server-to-server-interactions"
    },
    {
      "name": "7.1 Delivery",
      "id": "https://www.w3.org/TR/activitypub/#delivery"
    }
  ],
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#delivery",
      "branch": [
        7,
        1
      ]
    },
    "selector": {
      "type": "TextQuoteSelector",
      "prefix": "If a recipient is a Collection or OrderedCollection, then the server MUST dereference the collection (with the user's credentials) and discover inboxes for each item in the collection.\n",
      "exact": "Servers MUST limit the number of layers of indirections through collections which will be performed, which MAY be one.\n",
      "suffix": "Servers MUST de-duplicate the final recipient list.\n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}