If you are activated for military service, you can open a “uniformed services” TSP account and invest in it from your military pay. Investments may be made into traditional balances, Roth balances or a combination of both. (Note: Roth investments made into a uniformed services TSP account must be made based on a percentage of your military salary.)
Your military and civilian accounts will be combined for purposes of the annual investment limits.
When you return to your federal job, you may make up any investments to you federal employee TSP account you missed because of the military service, offset by any investments you made to a uniformed services TSP account.