Command Line — Every option, what it does, and the ones that surprise people — -O taking decimal only, -W turning on one sweep rather than all warnings, and the --no- forms that exist because a project file could otherwise only turn a setting on. Also init, the environment variables and the exit status.
The Project File — fantasm.toml: getting one, what each table does, and what happens when the command line and the file disagree. Covers [project], [assembler], [defines], [artifacts] and [tasks], the precedence rules, and the gotchas — including why a misspelt key is accepted in silence.
Output Formats — bin, hex, sna and nex — what each one is, which of the three places naming a format wins, and how to get code into a bank of a .nex so it is still there when the program runs.
Building Several Images — target, for a program that is more than one file: a payload and the loader that incbins it, or a release and a debug variant. Declaring targets, depends and build order, --only and --all, what happens when one fails, per-target recipes and per-target test counts.