Why Can an API Return HTTP 200 Before the Work Is Finished?

I’m integrating a REST API that starts a longer-running task, and I’m seeing an HTTP 200 response even though the result is not ready yet. The request seems to have been accepted, but the actual work continues afterward.

Is this a normal way to represent an asynchronous operation, or should I expect a different status code? I’m also unsure what the client should rely on to know when the task has completed: a job ID, a status endpoint, a callback, or something else.