When the plan needs cash, which balance it taps changes lifetime tax more than most return assumptions. The engine draws in a defined order, and you can change it.
The default order
Taxable → tax-deferred → Roth. Spend the least tax-advantaged dollars first and let Roth compound tax-free longest. One standing exception: a forced RMD always comes out of tax-deferred first. The IRS doesn't care about your preferred order.
Nothing else rearranges the order behind your back. If you want conversion-year spending funded from Roth, so living expenses don't consume the bracket headroom you're converting into, set that order yourself for the phase covering those years, under Money flow → Withdrawals.
What you control
- The draw order itself, per position in the sequence. One-time expenses (a down payment, a mortgage payoff) carry their own order, so a big single draw can come from a different pocket than daily spending.
- HSA balances, when present, sit outside the main order: qualified medical costs draw from HSA tax-free (displacing spending), and non-medical HSA draws after 65 are ordinary income.
"Withdrawal" here is the funding mechanic: which pocket the dollars leave. Your spending level is set in the plan; see spending vs. withdrawal for why the app keeps the two words apart.