You must be rehired to your previous position and status, or a position of similar seniority, status and pay, after you are activated for an emergency or other required military duty. If a disability you received during mobilization or deployment means you can no longer do that job, you must be offered a different position of similar seniority and pay that is within your capabilities.
There is nothing to apply for in advance. Tell your employer before you leave, give them your orders, and contact them to arrange reemployment when your service ends.
Your activation orders and your release from active duty paperwork, plus medical documentation if a service disability affects what work you can do.
