This is one of many protocol behaviors described in ActivityPub.

Identifier

urn:uuid:723f3629-b7ac-4c6e-8292-0b4d28c60e9d

Content

Identifiers MUST be provided for activities posted in server to server communication, unless the activity is intentionally transient.

Origin

prefix An ID explicitly specified as the JSON null object, which implies an anonymous object (a part of its parent context)

Identifiers MUST be provided for activities posted in server to server communication, unless the activity is intentionally transient.
 
suffix However, for client to server communication, a server receiving an object posted to the outbox with no specified id SHOULD allocate an object ID in the actor's namespace and attach it to the posted object.

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

JSON

{
  "id": "urn:uuid:723f3629-b7ac-4c6e-8292-0b4d28c60e9d",
  "type": "Behavior",
  "uuid": "723f3629-b7ac-4c6e-8292-0b4d28c60e9d",
  "name": "Identifiers MUST be provided for intransient activities posted in server to server communication",
  "content": "Identifiers MUST be provided for activities posted in server to server communication, unless the activity is 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": "Identifiers MUST be provided for activities posted in server to server communication, unless the activity is intentionally transient.\n",
      "prefix": "An ID explicitly specified as the JSON null object, which implies an anonymous object (a part of its parent context)\n",
      "suffix": "However, for client to server communication, a server receiving an object posted to the outbox with no specified id SHOULD allocate an object ID in the actor's namespace and attach it to the posted object. \n"
    }
  },
  "@context": [
    "https://www.w3.org/ns/activitystreams",
    "https://socialweb.coop/ns/testing/context.json"
  ]
}