The four triggers are not interchangeable, and two of them are restricted by who you are. The statute adds a sentence most summaries drop: the separation-from-service trigger "shall be applied only with respect to an individual who is an employee without regard to section 401(c)(1)," and the disability trigger "shall be applied only with respect to an employee within the meaning of section 401(c)(1)." Section 401(c)(1) is the provision that treats a self-employed person as an employee. So separation from service is available to a common-law employee and not to a self-employed owner, while disability is available to the self-employed owner and not to the common-law employee. A sole proprietor with a plan cannot use separation from service, because there is no employer to separate from; a rank-and-file employee cannot use the disability trigger. Both of them can use death or age 59½.
"Balance to the credit" is measured by kind of plan, not by account. Under section 402(e)(4)(D)(ii), all trusts that are part of a plan count as a single trust, and then "all pension plans maintained by the employer shall be treated as a single plan, all profit-sharing plans maintained by the employer shall be treated as a single plan, and all stock bonus plans maintained by the employer shall be treated as a single plan." Three buckets, aggregated separately. Two profit-sharing plans at one employer must both be emptied in the same tax year; a profit-sharing plan and a money purchase pension plan at that same employer do not have to be emptied together, because they fall in different buckets. Non-qualified trusts are excluded from the calculation, community property laws are disregarded, and amounts payable to an alternate payee under a qualified domestic relations order come out of the employee's balance entirely. The alternate payee's own balance can itself be treated as a lump-sum distribution, but only where the payee is the employee's spouse or former spouse and the employee's balance would have qualified.
The pre-1936 birth restriction is real but much narrower than it looks. Form 4972 offers two elections, 10-year averaging and a 20% capital gain treatment on the pre-1974 portion, and those are restricted to participants born before January 2, 1936. That restriction is easy to over-read, because it appears on the form that carries the phrase in its title. It gates the Form 4972 elections only. It is not part of the statutory definition in section 402(e)(4)(D), which contains no birth-year condition at all, and it does not reach the net unrealized appreciation election. The reason for the oddity is historical: those two elections descend from section 402(d), which Congress repealed in 1996, and they survive as a transition rule from the Tax Reform Act of 1986. They are not quite dead: someone born in 1935 turns 91 in 2026, and Form 4972 also lets a beneficiary of such a participant use them.
What actually happens by default. Absent a rollover, the taxable portion is ordinary income in the year received, at whatever marginal rate the sudden increase in income produces, and a workplace plan must withhold 20% for federal tax on an eligible rollover distribution paid to the participant. Anyone under 59½ without an applicable exception also faces the 10% additional tax. Rolling the money over avoids all of that, which is why the vast majority of lump-sum distributions become rollovers, and why the strict definition rarely gets tested.