When you join the station after a round has already started, you'll spawn with the assistant job. You can also choose to become an assistant by setting it as your job preference before the game starts.

Assistants start in the arrival shuttle with a grey jumpsuit, brown shoes, an assistan's id card and the usual pen, headset and backpack with an empty box in it.

When roleplaying, assistants are usually expected to report to the captain or head of personel for job assignment.