<subchar> is specified in a Unicode Character Map (UCM) file. They are used by enc2xs (part of Encode) to generate xs files to be compiled into an encoding. The documentation for enc2xs has a quick guide for making your own encoding.

If all you want to do is change the substitution character then it should be simple. Find the UCM file for the encoding you want to modify, change <subchar> to the value you want and follow the instructions.

Good luck!