This variable holds a list of directories that are used by package installers to add to certain other directories to determine where the default package system of Tcl is looking for packages. In contrast to auto_path, tcl_pkgPath is not intended to be used to find and load packages, but rather is intended to suggest to the default package system of Tcl locations where where new packages could be placed.
If you're not writing a package installer script, just pretend the tcl_pkgPath variable does not exist.
tcl_pkgPath may not exist on all platforms. At some point, tcl_pkgPath was removed from Tcl releases for some platforms, e.g., Microsoft Windows. In 2005, escargo reported that tcl_pkgPath was missing from the Cygwin port, but, at least in 8.5.11, it is there. See Tcl Bug 1072603.
LV 2007 Aug 24 So, if that is the case, perhaps library(n) should be updated to not indicate that this variable is used in the creation of the value of auto_path? And perhaps tclvars(n) should be updated to indicate the intent that it should only be used to locate Tcl's packages themselves? And maybe a warning in pkg_mkIndex(n) that the variable isn't intended to be used by non-package installers? Note that this variable is also mentioned in msgcat's man page.