When not to use Romex?

Do not use Romex anywhere wet or damp: outdoors, underground, in a conduit that leaves the building, or embedded in masonry. NM cable's paper-wrapped interior is dry-location only.

It is also out in most commercial and industrial occupancies, in air plenums, in exposed locations where it can be damaged, and in any jurisdiction whose local code requires conduit.

Above suspended ceilings and in attics it is fine when supported and protected; draped across joists near an attic hatch, it fails inspection.

The alternatives map cleanly: UF cable for wet and buried runs, THWN in conduit for outdoor and commercial work, MC cable where mechanical protection is required.

Not sure which cable a run calls for? The right wire size tool matches the wiring method to the location and load.