If your add-in files are not contained within a .jmpaddin file, you can use the Register Addin() JSL function to manually register the addin.def file. This installs and registers the add-in.
JMP might find a file named addin.def in the specified home folder. If so, values from that file are used for any optional arguments that are not included in the Register Addin() function.
The addin.def file is used only for values that are not provided in the Register Addin() function. This function is useful while developing, but not necessary, since the addin.def file is enough to register an add-in.