(a) Except as provided by Subsection (b) or Section 813.304(d), military service is creditable in a class of membership that includes a position held by the member who performed the service after the date of release from active military duty.
(b) Military service performed by a person who was a contributing member immediately before the date the member began military duty may be credited, at the option of the member, in the class of membership that includes the position held by the member immediately before the date the member began the military duty.