As a Tier 6 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 credit you earned as a member but for which you made no (or insufficient) contributions. You are required to pay the cost for such service at the rate you would have been assessed had you made timely contributions, plus any interest accrued. (See Contributing Toward Your Retirement.)
Example of Mandatory Past Service Costs:
You joined the Retirement System on May 14, 2012, but your employer did not begin taking contributions from your paycheck until July 1, 2012. You would then need to make payments for May 14th through June 30th plus any interest accrued.
Optional costs are payments you choose to make to purchase credit for a period of previous public employment or military service. You must accrue two years of service credit and make payment before you can receive prior service credit. Five years of service is required before you may purchase most military service.