The Principal Register offers the strongest federal protection for your trademark. It affords you all of the benefits of Federal Registration. Marks allowed on the Principal Register are ones that are either arbitrary/fanciful marks or marks that have may be descriptive, but have obtained a secondary meaning. Sometime when denied the Principal Register a mark is given the option to be placed on the Supplemental Register. This occurs when a mark is descriptive, but has not yet acquired a secondary meaning. The Supplemental Register gives these marks time to acquire a secondary meaning while giving them certain benefits. The Supplemental Register allows the use of the registered symbol ® and can act as a deterrent from those that would infringe on your use or attempt to register a similar mark. It also allows suit to be brought in federal court and can be used as a basis for certain foreign applications.
This blog is not legal advice and is not specific to your application. You should always consult an attorney.