The USABDA rule book (see http://usabda.org/dancesport_competitors/rules-policies-bylaws/dancesport_rulebook/index.cfm) talks about this in section 4.5.3, and even gives a small diagram -- although admittedly even with this it's still confusing.
To make a long story short, syllabus and Novice sit alongside each other. So, if you're doing syllabus, you can also do Novice, and if you're doing closed syllabus work on the Novice level, you can also do syllabus. Furthermore, rule 4.5.4.6 b) states "If one or both of the proficiency levels is a Syllabus level, then a couple may also enter the Novice level."
If you are doing Silver, you are a syllabus dancer who is also eligible for Novice. The next level up for Silver dancers who are doing syllabus would normally be Gold. At any time that you are dancing Syllabus, you can also enter Novice if a Novice category is offered.
All this confusion stems from how dance competitions were commonly run 10 or more years ago in the US. On the West Coast there were no syllabus divisions, the lowest competitive level was called Novice. On the East Coast there were syllabus competitions, so people would work their way up through the syllabus levels and then go in to Pre-Champ. Rather than picking one system over the other, USABDA attempted to merge the two, thus resulting in ongoing confusion to all newcomers

In practice, Novice is really becoming the level between Gold and Pre Championship. Personally, since so many people think of it that way, I think the rules should be cleaned up and simplified so that rather than Novice being alongside that it gets put "in line" where people expect it to be, but that's just my opinion.
If you have any other questions please ask, I'm quite familiar with the rule book and what the rules mean because I've been working as a competition Registrar for some USABDA competitions for the past couple of years.