Returned To Unit or RTU refers to a military member being returned to their home base or home unit, either due to their being medically unfit, their requesting to be withdrawn from training, or their being unfit for training or otherwise disorderly.
As military members can only be court-martialed at their home base or unit, being RTU'd for misconduct can often be a precursor to further judicial punishment.
RTU'ing is a concept used during a period of specialist training, whereby a service person has been extracted or detached from their home unit.
For example, if a service person has been extracted to complete an instruction course run by a specialist unit within the armed forces such as a PTI or Range Control course.
The reason this is done is that, according to military law, disciplinary action such as a court-martial can only by issued against a service person by their home unit.