A mutable hash table containing user-defined paths to external programs used by Macaulay2. Its keys are strings containing the names of programs and must coincide with the first argument of findProgram.
It is only necessary to define a path in this way if a program is installed in a non-standard location. In particular, findProgram already checks prefixDirectory | currentLayout#"programs", (where the programs shipped with Macaulay2 are installed) and all of the directories in the user's PATH environment variable.
If you use a particular program frequently and it is installed in a non-standard location, then it may be useful to add a line to your initialization file defining its path in this way.