Condign, Congruous and Supererogatory Merit

Ok, I'm not Roman Catholic, but I'm pretty familiar with the doctrine. The following fits with what I know of the Roman Catholic view of works and salvation, but I've never seen these specific terms used before. Any Catholics out there who can enlighten me? If this is correct, it explains a lot.

Catholics speak of three types of merit, each of which plays a role in salvation:

• Condign Merit. This is merit attributed to our works for which God is obligated to give reward. This is like paying a laborer his due wages.
• Congruous Merit. This is merit that is “reasonable”, but not obligated. In secular terms, it is something like a waitress’ tip. It is attained through works and penance.
• Supererogatory Merit. This is the stuff of saints. It is their “excess” merit and it is deposited in a treasury of supererogatory merits. It can then be drawn upon to free people from purgatory. Attaining supererogatory merit is also possible for a priest living a life of celibacy in devotion to Christ. A layman can accrue supererogatory merit through regular church attendance and constant attention to the sacraments. Mary is thought to have contributed enormous excess merit into the treasury.