Ok, i wanted to post this because i've gotten many questions about it lately.
Since the drop of froyo source, there are a lot of ROMs flying around with build ids on them like FRF57 and FRF83. I'm going to clear up what these ids mean and how they are significant.
Basically, a build id is just that, it's an identifier that lets a developer know that for any given piece of software in the field, what build of the code it is. This helps them identify problems.
Although I have not found any definitive proof of how the first three letters of the build ids are generated, they tend to relate to what version of android is being ran as well as it's stability. I would guess something along the lines of:
A = Android codename (C = cupcake, D = donut, E = eclair, F = froyo, etc)
B = Stability (R = release-candidate, S = stable)?
C = ?? no idea on this one, possibly just a letter increment
XX = build number
So for instance, FRF57 is the 57th build of a release candidate for froyo.
So back to the difference of FRF57 and FRF83. FRF57 is the build id for the latest leak from motorola. An FRF83 leak for the Droid DOES NOT EXIST. FRF83 is the build id for the nexus one release.
Now, the numeric build number is DEVICE SPECIFIC. Just because the nexus build is FRF83 and the motorola build is FRF57 DOES NOT mean that the nexus build is necessarily more stable or more advanced. All it means is that nexus has built their source 83 times, whereas motorola has built theirs 57 times.
Also, it is important to know that for source builds, the source itself is IN NO WAY RELATED TO THESE BUILD IDS. The only exception is for some tagged sources, which reference device-specific source, but that does not currently exist for froyo.
So what does this all mean?
Many source ROMs will be released in the following weeks that will label themselves FRF57. This means that they use the PROPRIETARY files from the FRF57 leak. The source is still the latest source.
Some ROMs will label themselves FRF83. Although I am not certain this only means that SOME proprietary apps, were taken from Nexus and placed on the Droid. The actual hardware drivers on these ROMs are STILL FRF57. Also it is important to remember what was stated above about the numeric portion of a build id being device-specific. Although the ROM may pull device-independent apps from nexus, that doesn't guarantee that they are any more advanced than the motorola ones. (They may be, but it's not a guarantee)
Since the source itself as well as the features implemented are not related to these proprietary files, as of Sapphire 0.6.1 I will be dropping the build ID labeling from the release page, and the reference to these proprietary files will be available in the changelog and build.prop, but not be used to identify the ROM.
I feel that this is the right thing to do, as personally I think it is somewhat of a lie to users to label a ROM FRF83 when the core hardware files are still FRF57, especially when most users don't fully understand what those build numbers mean, other than that one number is higher than the other. Also, pairing a source ROM with a leaked id, confuses users into whether a ROM is truly built from source or not.
I hope that this has cleared up some confusion about all of this.