All three of them refer exclusively the RAM.
$rpacor is special for the rirpa, $ricore for ridft, rdgrad, escf?, egrad?, rirpa?, aoforce? and might be replaced in the future by $maxcor
$maxcor is the orginal variable for the max. amount of RAM used by the TURBOMOLE programs for dynamic memory allocation in the Fortran code.
Note that on top of that the binaries need some memory themself and on Linux the MKL library, which is used for the linear algebra stuff, allocates
internally some extra RAM and the operating system needs some RAM to buffer I/O.
As a rule of thumb, we recommend to set $maxcor to about 66 - 75% of the physically available RAM, if you don't have a better estimate.
ricc2, ccsdf12, pnoccsd, mpgrad and mpshift use exclusively $maxcor. They ignore $rpacor and $ricore.