Limb one: a change in the ownership of a corporation. This occurs on the date one person, or more than one person acting as a group, acquires stock that, together with stock already held, constitutes more than 50 percent of the total fair market value or the total voting power of the corporation's stock. A plan may set a higher percentage than 50, provided it does so by the deadline for fixing the time and form of payment. Two limits follow from the text. Someone already over the line cannot trigger the event by buying more, and the limb applies only where stock is transferred or issued and stock remains outstanding afterwards, which is what separates it from the asset limb.
Limb two: a change in the effective control of a corporation. Two quite different things satisfy it. The first is a person or group acquiring, in one go or across the 12-month period ending on the date of the most recent acquisition, stock with 30 percent or more of the corporation's total voting power. The second is the replacement of a majority of the board of directors during any 12-month period by directors whose appointment or election was not endorsed by a majority of the board sitting before that appointment. A proxy contest that turns over the board therefore counts, with no shares changing hands and no transaction of any kind. As with the first limb, a plan may substitute a higher percentage or a larger portion of the board, and a person who already has effective control does not trigger it again by acquiring more.
Limb three: a change in the ownership of a substantial portion of the assets. This occurs when a person or group acquires, in one transaction or over the 12-month period ending on the most recent acquisition, assets with a total gross fair market value equal to at least 40 percent of the total gross fair market value of all the corporation's assets immediately before the acquisition. "Gross" is doing work: the regulation says value is determined without regard to any liabilities associated with the assets, so a heavily mortgaged division counts at its full value. There are carve-outs for transfers that keep the assets in the family: a transfer to a shareholder in exchange for its stock, to an entity the corporation owns half or more of, to a person owning half or more of the corporation, or to an entity half or more owned by such a person, is not a change in the ownership of those assets.
Which corporation has to have the event. The regulation does not accept any corporate change anywhere in the structure. The event must relate to the corporation the service provider works for, or the corporation liable for paying the deferred compensation, or a corporation that is a majority shareholder of one of those, following the chain upward. Its own worked illustration has a parent, a subsidiary and a sub-subsidiary: a change at the middle company is an event for people working at the middle or bottom, and is not an event as to the parent unless the sale is itself a change in the ownership of a substantial portion of the parent's assets.
Ownership is counted with attribution, and options count in a specific way. Section 318(a) applies to determine stock ownership for all three limbs. Stock underlying a vested option is treated as owned by the option holder; stock underlying an unvested option is not. There is one exception: where a vested option is exercisable for stock that is not substantially vested, the underlying stock is not treated as owned. The "acting as a group" test is narrower than it sounds. People are not a group merely because they bought stock at the same time or in the same public offering. They are a group if they are owners of a corporation entering into a merger, consolidation, stock purchase or similar transaction with the corporation in question.
One interaction worth flagging. Ordinarily a forfeiture condition cannot be extended in order to keep compensation deferred. The regulation makes a narrow exception where a condition would otherwise lapse because of a change in control event under the ownership or asset limb: it may be extended or modified before and in connection with that event, and it will still be treated as a substantial risk of forfeiture, provided the transaction is a bona fide arm's length deal with unrelated parties and the extended condition would itself qualify. That is what lets an unvested award keep its deferred status when a buyer re-sets the vesting condition as part of the deal.