What type of discharge or separation from service is required for an employee to be entitled to reemployment under USERRA?

What types of discharge or separation from uniformed service will make the employee ineligible for reemployment under USERRA?

Who determines the characterization of service?

If the employee receives a disqualifying discharge or release from uniformed service and it is later upgraded, will reemployment rights be restored?

If the employee receives a retroactive upgrade in the characterization of service, will that entitle him or her to claim back wages and benefits lost as of the date of separation from service?