From Online Manual
Difference between security release and a public release
Now that SMF 2.0 it is near the gold release, it has been a major update to 2.0 RC5 and also a security patch. Some people might be wondering what is the difference between both.
For a start, a security patch it is only for fixing major bugs or issues that can be caused in outdated SMF versions. The only thing it does it is to fix those bugs and keep SMF the less bugfree.
But the 2.0 RC5 version fix those bugs and many things, like new functions and development tools for mod and themes creators to use (just like in the previous release 2.0 RC4, where the integrations hook were added) and/or other minor bugs fix.
So, to make a resume: A security patch it is only to fix major bugs and a public release contains also major bugs fixing and new code/functions or minor bugs fixing.