This is one of many protocol behaviors described in ActivityPub.

Identifier

urn:uuid:66d5b416-7660-48c8-884b-fda5e26b28e8

Content

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.

Origin

prefix The inbox is determined by first retrieving the target actor's JSON-LD representation and then looking up the inbox property.

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

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

JSON

{
  "id": "urn:uuid:66d5b416-7660-48c8-884b-fda5e26b28e8",
  "uuid": "66d5b416-7660-48c8-884b-fda5e26b28e8",
  "type": "Behavior",
  "content": "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",
  "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": "The inbox is determined by first retrieving the target actor's JSON-LD representation and then looking up the inbox property.\n",
      "exact": "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",
      "suffix": "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/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}