AIX determines the OS Level by comparing the installed filesets to a list of known APARs. This means that OS levels can change by installing new packages.
The OS level can be in the following formats:
<OS Ver>-<TL>-<SP>-<BUILD DATE>
or
<OS Ver>-<TL>-<SP>
Example:
6100-06-07-1207 is AIX 6.1 TL 6 SP 7 and was built / released on the 7th week of 2012
Note that the AIX download cacher tool uses the first format.
Build dates are also present on filesets. They are a 4-digit number that represent the year and week of the year that the product was built.
Build dates are important because, in most cases, the AIX operating system will not allow products to be replaced by a version with an earlier build date. For example, AIX 6100-06-07-1207 cannot be patched to 6100-07-02-1150 because 6100-07-02-1150 is an earlier release.