Returns the installation directory for a specified application. If name is omitted or the null string, then the default custom directory is returned, either the first value of the APTCUSTOM environment variable if set or else the custom subdirectory in the product installation directory.

If name is a number, then this specifies the 0-based index into the list of custom directories. This allows an iterator to enumerate the list of custom directories by calling this method in a loop, incrementing the index until a null string is returned.

If name is a negative integer, then the list is traversed in reverse. "-1" returns the last custom directory, "-2" the second to last custom directory and so on.