This is one of many protocol behaviors described in ActivityPub.

Identifier

urn:uuid:08ed852d-f36c-47ba-992b-b211e35dbdca

Content

Origin servers sending publicly addressed activities to sharedInbox endpoints MUST still deliver to actors and collections otherwise addressed (through to, bto, cc, bcc, and audience) which do not have a sharedInbox and would not otherwise receive the activity through the sharedInbox mechanism.

Origin

prefix Additionally, if an object is addressed to the Public special collection, a server MAY deliver that object to all known sharedInbox endpoints on the network.

Origin servers sending publicly addressed activities to sharedInbox endpoints MUST still deliver to actors and collections otherwise addressed (through to, bto, cc, bcc, and audience) which do not have a sharedInbox and would not otherwise receive the activity through the sharedInbox mechanism.
 
suffix 7.2 Create Activity

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

JSON

{
  "id": "urn:uuid:08ed852d-f36c-47ba-992b-b211e35dbdca",
  "uuid": "08ed852d-f36c-47ba-992b-b211e35dbdca",
  "type": "Behavior",
  "content": "Origin servers sending publicly addressed activities to sharedInbox endpoints MUST still deliver to actors and collections otherwise addressed (through to, bto, cc, bcc, and audience) which do not have a sharedInbox and would not otherwise receive the activity through the sharedInbox mechanism.\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": "7.1.3 Shared Inbox Delivery",
      "id": "https://www.w3.org/TR/activitypub/#shared-inbox-delivery"
    }
  ],
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#shared-inbox-delivery",
      "branch": [
        7,
        1,
        3
      ]
    },
    "selector": {
      "prefix": "Additionally, if an object is addressed to the Public special collection, a server MAY deliver that object to all known sharedInbox endpoints on the network.\n",
      "exact": "Origin servers sending publicly addressed activities to sharedInbox endpoints MUST still deliver to actors and collections otherwise addressed (through to, bto, cc, bcc, and audience) which do not have a sharedInbox and would not otherwise receive the activity through the sharedInbox mechanism.\n",
      "suffix": "7.2 Create Activity\n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}