In the context of restricted stock units or performance share units, this rule allows FICA taxes to be collected with federal income tax as long as this occurs in the same calendar year in which the employee has a legal binding right to the stock (not at the vesting date or the share-delivery date), or no later than a brief time after the end of the calendar year (2.5 months, similar to the short-term deferral period under Internal Revenue Code Section 409A). When companies use this rule, they need to follow it for all employees participating in the plan and for all similar plans.