Subagent
A second agent an agent starts for one bounded job, with its own context, so the work does not fill the caller's.
The reason is arithmetic, not tidiness. Reading forty files to answer one question costs the whole context of whoever reads them; a subagent reads them in its own, and returns the answer instead of the files. The caller keeps room to finish the task.
That isolation is also the cost. A subagent knows only what it was told, so a vague brief comes back with a confident answer to a different question. The instruction has to carry everything the job needs, because there is nothing to ask back to.
It is worth several at once when the pieces are genuinely independent — separate directories, separate dimensions of a review — and worth nothing when the second step needs the first step's answer.