This is one of many protocol behaviors described in ActivityPub.

Identifier

urn:uuid:fc2bfa75-8fd0-45e0-809d-7aa3970be8ae

Content

all objects distributed by the ActivityPub protocol MUST have unique global identifiers, unless they are intentionally transient

Origin

prefix All Objects in [ActivityStreams] should have unique global identifiers. ActivityPub extends this requirement;

all objects distributed by the ActivityPub protocol MUST have unique global identifiers, unless they are intentionally transient (short lived activities that are not intended to be able to be looked up, such as some kinds of chat messages or game notifications).
 
suffix These identifiers must fall into one of the following groups:

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

JSON

{
  "id": "urn:uuid:fc2bfa75-8fd0-45e0-809d-7aa3970be8ae",
  "type": "Behavior",
  "name": "intransient objects must have unique global identifiers",
  "content": "all objects distributed by the ActivityPub protocol MUST have unique global identifiers, unless they are intentionally transient\n",
  "origin": {
    "source": "https://www.w3.org/TR/activitypub/",
    "section": {
      "id": "https://www.w3.org/TR/activitypub/#obj-id",
      "branch": [
        3,
        1
      ]
    },
    "selector": {
      "type": "TextQuoteSelector",
      "exact": "all objects distributed by the ActivityPub protocol MUST have unique global identifiers, unless they are intentionally transient (short lived activities that are not intended to be able to be looked up, such as some kinds of chat messages or game notifications).\n",
      "prefix": "All Objects in [ActivityStreams] should have unique global identifiers. ActivityPub extends this requirement; \n",
      "suffix": "These identifiers must fall into one of the following groups:\n"
    }
  },
  "uuid": "fc2bfa75-8fd0-45e0-809d-7aa3970be8ae",
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}