I'm curious how they come up with the build numbers on the "special" builds. So, 13B42 should be the 42nd build of 10.9("13").1("B"), just like 10.9.0 was 13A603 and the 603rd build of 10.9.0.
I have a 2013 rMBP that came with 13A2093 (2093rd build of 10.9.0?) and is now running 13B3116 (3116th build of 10.9.1?). All of the leaked Mac Pro benchmarks show 13A4023 (4023rd build of 10.9.0?), as far back as Sept 27th - but that build is much further along numerically than the released build for the rMBP leaked on Oct. 23rd!
Anyone who understands how this works, please chime in.
Special variants (typically model-specific) start at a build number which is some multiple of 1000, with lower build numbers reserved for the common variant. I think that in recent OS X versions, Apple has started special build numbers at 2000, leaving 0-1999 available for the common variant.
Therefore 13A2093 is build 93 of special variant 2 of 10.9.0, 13B3116 is build 116 of special variant 3 of 10.9.1, and 13A4023 is a build 23 of special variant 4 of 10.9.0.
The build number usually gets reset (presumably to 1) each time the major number or minor version letter increases.
The letter usually corresponds to minor versions (e.g. 13A and 13B are 10.9.0 and 10.9.1 respectively), but that isn't always the case. Strictly, the letter increases when the kernel changes: in some minor OS X versions, the kernel doesn't change (so consecutive minor versions have the same letter), and sometimes it jumps more than one letter between minor OS X versions (e.g. 10.7.4 was 11E53, and 10.7.5 was 11G56).
10.6 had an odd pattern: the build numbers of 10.6.1 and later were all in the 500-900 range, and it looks like Apple was using a different internal system to number the builds. The build number for released versions increased from 10.6.0 to 10.6.3 despite kernel version letter increases, then went down slightly in 10.6.4, up again in 10.6.5, down slightly in 10.6.6 and down again in 10.6.8 (10.6.7 was the same kernel version as 10.6.6).
10.4 was a special case, since it had parallel releases for PowerPC and Intel. The Intel builds used build numbers 1000+ or 2000+ while the PowerPC builds used lower numbers, but otherwise the standard rules applied.
For other major versions of OS X, the 10.x.1 and later releases usually have build number in the 1-99 range (sometimes getting up into the 200s) and adjacent versions have unrelated build numbers (unless they have the same kernel version letter, in which case the newer OS X version has a higher build number than the previous version).
