Help is layered, and the layers are in the order you should try them.
This manual, at the point of need. Sections of this manual are linked from the screens they describe, opening at the section — not the cover — that answers the screen's question. When you are stuck on a field or a workflow, that link is the fastest route to the relevant explanation, including the "settings that shape this module" section that tells you why the screen behaves as it does in your company.
Setup Progress, for "why is this missing?" If something you expect is absent — a setup step, a category, a warehouse — the checklist at System Setup → Setup Progress (§0.9) shows what your company has and has not filled in yet, and links you to the gap.
The Activity trail, for "what happened here?" Every record's Activity tab (§1.4) and the document's own history answer who did what and when — most mysteries about a changed figure are solved there, by the people who were involved, without escalating.
Your company administrator, for everything else. Permissions, switches, modules, and roles are theirs to change; this manual's Chapter 16 is written for them, and the Implementation Manual sits behind it for configuration detail. Asking with the document number or record code in hand (SO-1042, CUS-001) gets you a faster answer than any description — codes are how the application itself names things.
See also: About this manual · 0.9 Setup Progress — knowing where you stand · 16.5 The activity trail — who did what, and when