In 7.1.4.1 "Definitions",
there's the text:
    Each task [...] is associated with a Document;
    if the task was queued in the context of an element, then [...];
    if the task was queued in the context of a browsing context, then [...];
    if the task was queued by or for a script then [...].

This appears to assume that we can detect when each of those three possibilities holds, but it's not clear how.

The only other place in the spec that connects tasks with the phrase "in the context of" is in resource selection algorithm (4.8.10.5), where step (10).10 says:
    Failed: Queue a task, using the DOM manipulation task source,
    to fire a simple event named error at the candidate element,
    in the context of the fetching process that was used to try
    to obtain candidate's corresponding media resource in the
    resource fetch algorithm.
But:
(1) It's unclear whether "in the context of" modifies "Queue a task" or
    "fire a simple event".
(2) Even if it modifies "Queue a task", it says "in the context of the
    fetching process", which doesn't appear to match any of the three
    possibilities raised above.

[By the way, in the resource selection algorithm, where it says:
    "Otherwise, the source elements will be used; run these substeps:"
that should probably be made a separate step (<li>).]