If your income exceeds spending in a year - whether during your working years or any saver year in retirement - the leftover cash is routed in two steps:
- Cash reserve top-up. If you've set a reserve target on a cash account and the current balance has been drawn below that target, the surplus first refills cash up to the target. This mirrors how a real person treats an emergency fund - replenishing it before investing further.
- Taxable investment. The remainder is deposited into your Taxable investment account. Its cost basis is bumped by the deposit so the future capital-gains math doesn't treat the surplus as appreciation.
If you don't have a Taxable account in your plan, the entire surplus (after any cash top-up) lands in Cash instead.
Tax-advantaged accounts (Roth, Tax-deferred, HSA) never receive automatic surplus deposits. They only grow through their dedicated contribution path on the Work & Other page, which respects the annual IRS limits and contribution-vs-earnings tracking those accounts require.