The Uniform Services Employment and Reemployment Rights Act (“USERRA”) requires that a USERRA leave be given the “most favorable treatment” to any “comparable leave” provided by the employer.  20 C.F.R. § 1002.150.  Since most USERRA leaves are unpaid, this “most favorable treatment” requirement raises questions when an employer offers different types of paid leave.  If an employer gives two days’ paid leave for a bereavement leave, does an employee on a USERRA leave have the right to two days’ pay?
The key concept is “comparability” – an imprecise term, indeed.  USERRA does provide some refinement:

If the non-seniority benefits to which employees on furlough or leave of absence are entitled vary according to the type of leave, the employee must be given the most favorable treatment accorded to any comparable form of leave when he or she performs service in the uniformed services. In order to determine whether any two types of leave are comparable, the duration of the leave may be the most significant factor to compare. For instance, a two-day funeral leave will not be “comparable” to an extended leave for service in the uniformed service. In addition to comparing the duration of the absences, other factors such as the purpose of the leave and the ability of the employee to choose when to take the leave should also be considered.

20 C.F.R. § 1002.150 (emphasis added).  Thus, points of comparability are the duration, purpose, and timing of other leaves.  While a paid sabbatical leave may be comparable, a two-day bereavement leave is not.  But the USERRA comparability requirement is far from fully developed.  Failure to comply can result in a complaint to the U.S. Department of Labor and the risk of enforcement penalties.
Takeaway:  An employer who voluntarily provides pay for otherwise unpaid leaves needs to keep an eye on the USERRA comparability requirement.  A good resource for checking your thinking on comparability is the web page and helpline for the U.S. Department of Defense, Employer Support of the Guard and Reserve Office (ESGR).