As a Tier 5 member, you will be required to pay for service currently not credited to you. There are two kinds of past service costs — mandatory and optional.
Mandatory costs are required for service you earned as a member but for which you made no (or insufficient) contributions.
Example of Mandatory Past Service Costs: You joined the Retirement System on January 14, 2010, but your employer did not begin taking contributions from your paycheck until February 1, 2010. You would then need to make payments for January 14th through January 31st.
Optional costs are payments you choose to make to purchase credit for a period of previous or military service. Once you have made payment and have accrued two years of service credit as a member, you can receive that credit. Before you purchase optional past service credit, though, you must pay for any outstanding mandatory costs.