W-2 Compensation: What does it really mean?

According to a Technical Update posted on the SungardRelius.com website, "the short answer is that the W-2 definition reflects Box 1 of Form W-2, "Wages, tips, and other comp." This is the number a taxpayer uses to complete Form 1040. However, for 415 purposes (and for the purposes of the various Code sections which are based on the 415 definition of compensation, such as the top-heavy rules), the [retirement] plan must add back in elective deferrals."  The Update provides further explanation on including deferrals, handling of Roth deferrals and 'exceptions to the rules' to keep in mind when calculating W-2 compensation.