What do "weathered" and "custom modifications" mean?

These are two flags you can set on a piece to note that it isn't strictly factory-original.

• Weathered — the model has been deliberately aged with paint/powders to look realistically used (rust, grime, fading). Common and often desirable, but it's not how it left the factory, so it's worth flagging.
• Custom modifications — any other changes from stock: added details, upgraded electronics or sound, repaints, kitbashing, new couplers, and so on.

Why flag them?

Originality matters to collectors and to value. Marking these makes your record honest and helps anyone viewing a public piece — or a potential buyer — understand exactly what they're looking at. You can also filter your collection by these flags.

See What can I record about a piece? for the full list of details you can track.