Documentation for AIX OS version

"oslevel -s" output may say the system is older than it actually is.
It by default reports the most recent consistent level. If a single
fileset is missing  (for example /usr/include files)  at the latest
level, the old level is reported rather than the level of "most" of
the system.

$ oslevel -s
6100-09-03-1415
$ oslevel -q -s | head -3
Known Service Packs
-------------------
6100-09-03-1415
6100-09-02-1412
6100-09-01-1341

The RD seems to be important for finding filesets:
bos:bos.rte.install:6.1.9.16: : :C:F:LPP Install Commands: : : : : : :0:0:/:1415