This is one of many protocol behaviors described in ActivityPub.

Identifier

urn:uuid:7f16b1b5-6c95-4d4c-83f1-3a3962a2e37c

Content

When objects are received in the outbox..., the server MUST target and deliver to: The to, bto, cc, bcc or audience fields if their values are individuals or Collections owned by the actor.

Origin

prefix 7.1.1 Outbox Delivery Requirements for Server to Server

When objects are received in the outbox (for servers which support both Client to Server interactions and Server to Server Interactions), the server MUST target and deliver to: The to, bto, cc, bcc or audience fields if their values are individuals or Collections owned by the actor.
 
suffix These fields will have been populated appropriately by the client which posted the Activity to the outbox.

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

JSON

{
  "id": "urn:uuid:7f16b1b5-6c95-4d4c-83f1-3a3962a2e37c",
  "uuid": "7f16b1b5-6c95-4d4c-83f1-3a3962a2e37c",
  "type": "Behavior",
  "content": "When objects are received in the outbox..., the server MUST target and deliver to: The to, bto, cc, bcc or audience fields if their values are individuals or Collections owned by the actor.\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"
    },
    {
      "name": "7.1.1 Outbox Delivery Requirements for Server to Server",
      "id": "https://www.w3.org/TR/activitypub/#outbox-delivery"
    }
  ],
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#outbox-delivery",
      "branch": [
        7,
        1,
        1
      ]
    },
    "selector": {
      "type": "TextQuoteSelector",
      "prefix": "7.1.1 Outbox Delivery Requirements for Server to Server\n",
      "exact": "When objects are received in the outbox (for servers which support both Client to Server interactions and Server to Server Interactions), the server MUST target and deliver to:\n  The to, bto, cc, bcc or audience fields if their values are individuals or Collections owned by the actor.\n",
      "suffix": "These fields will have been populated appropriately by the client which posted the Activity to the outbox. \n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}