Currently the VARS command is the main source of info; acknowledged that VARS is lacking/cryptic (some meanings are obvious, many are not). This is an improvement area on our list but I welcome you to submit an Idea to accumulate votes.
In the meantime there is some additional info on VARS beyond the limited Description field that may help for some variables, mainly in the MVS and CICS area:
For CICS the DFHGROUP + MCT fields show the CICS source field for most of the transaction-level variables.
e.g. variable CPUTIME comes from CICS MCT monitoring field DFHTASK 8.
For MVS variables that have a value of MVS-xxxxx in the Location field, xxxxx is the name of the source MVS control block and the offset of the source field in that control block is in the MOMF field. (Location values other than MVS-xxxxx refer to internal SYSVIEW collection blocks which will have no meaning to you.)
e.g. ASITOTAL comes from MVS block ASVT+204, which is ASVTMAXU (max number of address spaces on system).
Cryptic? Absolutely. VARS shows the internal variable definitions that are used to collect the data and wasn't intended to serve as external documentation, but is currently filling that gap in lieu of not much else in this area. For those MVS variables it would minimally be nice to show the dsect field name rather than just the offset.
For the subset of MVS JOBxxxx variable values that are shown on the ASPERF command, HELP ASPERF shows the variable name and the MVS source field name(s). This is the type of info that needs to be available for all variables I imagine...
For help with any other specific variables that you are having trouble deciphering it would be best to open a case with support at support.ca.com