First 2 letters are the region and carrier.
Third letter is the year, fourth letter is the month, fifth letter is the version of that month.
I may have it wrong, but that's the way I always read it.
UCJF6 of the captivate was the original firmware, from July 2010.
J is the tenth letter, hence 2010.
F is the sixth letter, hence the sixth month.
6 means the sixth revision in that month.
They then released JH7, from September 2010.
J is 2010, H is September (8th month), 7 was the revision.
Iphone 3G > T-Mobile G1 > Samsung Captivate > Samsung Focus > Samsung Galaxy SII> Nokia Lumia 920 > HTC 8X > Sony Xperia ZL
...and a bunch of other phones in between