401(k), 403(b), Traditional IRA, Roth IRA, Rollover IRA, taxable accounts, pension if there is one. Each carries different tax treatment, different withdrawal rules, and different roles inside the retirement plan. The work is using each one for what it does best, rather than running them as separate buckets.
Retirement isn't a date, it's 25 to 40 years of changing conditions. The plan gets tested against bad market scenarios, accelerating healthcare costs, tax-law changes, and longevity beyond what the household expects.
Social Security, pension elections, IRA withdrawals, Roth conversions, RMDs, and portfolio sales each carry their own tax math. Pulled in the wrong order, they push the household into brackets it didn't have to enter. The plan sequences each source against the bracket math, year by year, across the whole retirement.
Early retirement, mid-retirement, and late retirement each have their own math. Spending looks different across the stages, medical costs rise, RMDs activate at 73, and surviving-spouse brackets change. The plan gets reopened as each stage arrives.