"...With the introduction of Roth, you will potentially have two types of balances in your TSP account: A traditional (non-Roth) TSP balance and a Roth TSP balance. Any agency contributions you receive will always be a part of your traditional (non-Roth) balance. However, you may designate your own contributions any way you like depending on your individual tax circumstances. (Note: Money already in your account when you begin making Roth contributions will remain part of your traditional balance. You will not be able to convert it to Roth.)
The table below compares the treatment of the two different types of contributions..."
Read more at tsp.gov
