Mode decides how a request is approached, independently of how far it reaches.
Default
Plan the work, do it, return it. The right choice almost always.
Simulation
Default, plus a discovery pass before anything is selected: the run goes looking for what is relevant — beyond what your request named and what the directory already holds — before deciding what to build. Use it when you suspect the interesting material is something you have not thought to mention.
It is slower and reads more, so it costs more.
Ask
Does not run the pipeline at all. Your question goes straight to the organisation model and is answered from what is already known — fast, cheap, and no new artefacts. Use it when the answer should already be in the model and you want it stated.
Choosing
Mode sits next to build scope in the composer. Simulation is included on some plans and not others — see Plans.