When the environment variable TCL_LIBRARY is defined, Tcl's default initialization sequence takes its value to be a filesystem directory in which the init.tcl file should be sought first. In a proper Tcl installation, use of this environment variable should not be needed. The main use for it is in Tcl's own build and test procedures to direct initialization of a freshly built Tcl program so that it can be tested before it is installed. Other than that, it should be considered a tool for "last-resort" workarounds of broken installations, not a tool for routine use.
See also magic names, pkgIndex.tcl, package require, auto_path