This is one of many protocol behaviors described in ActivityPub.

Identifier

urn:uuid:5e781a3a-4314-4ec6-b973-717456dc5da5

Content

The server MUST only target the values of to, cc, and/or audience on the original object being forwarded, and not pick up any new addressees whilst recursing through the linked objects (in case these addressees were purposefully amended by or via the client).

Origin

prefix The server SHOULD recurse through these values to look for linked objects owned by the server, and SHOULD set a maximum limit for recursion (ie. the point at which the thread is so deep the recipients followers may not mind if they are no longer getting updates that don't directly involve the recipient).

The server MUST only target the values of to, cc, and/or audience on the original object being forwarded, and not pick up any new addressees whilst recursing through the linked objects (in case these addressees were purposefully amended by or via the client).
 
suffix The server MAY filter its delivery targets according to implementation-specific rules (for example, spam filtering).

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

JSON

{
  "id": "urn:uuid:5e781a3a-4314-4ec6-b973-717456dc5da5",
  "uuid": "5e781a3a-4314-4ec6-b973-717456dc5da5",
  "type": "Behavior",
  "content": "The server MUST only target the values of to, cc, and/or audience on the original object being forwarded, and not pick up any new addressees whilst recursing through the linked objects (in case these addressees were purposefully amended by or via the client).\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": "7.1.2 Forwarding from Inbox",
      "id": "https://www.w3.org/TR/activitypub/#inbox-forwarding"
    }
  ],
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#inbox-forwarding",
      "branch": [
        7,
        1,
        2
      ]
    },
    "selector": {
      "prefix": "The server SHOULD recurse through these values to look for linked objects owned by the server, and SHOULD set a maximum limit for recursion (ie. the point at which the thread is so deep the recipients followers may not mind if they are no longer getting updates that don't directly involve the recipient).\n",
      "exact": "The server MUST only target the values of to, cc, and/or audience on the original object being forwarded, and not pick up any new addressees whilst recursing through the linked objects (in case these addressees were purposefully amended by or via the client).\n",
      "suffix": "The server MAY filter its delivery targets according to implementation-specific rules (for example, spam filtering).\n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}