Packages documentation

Hello I see in the reference Max -> Guides -> About Packages there is a list of objects/folders/files for packages. Where is the specific documentation about e.g. the default-definitions folder in terms of what can go in there, how to configure etc?

I'm looking to confirm where it is defined as to what form max definesubstitution takes as a directive in a file in the init directory of a package. I only already now know this from a youtube video where some one else has already done it, but I'm looking into where it says, 'Files in the init directory take the form blah for example blah' etc. - does this even exist?

I note that definesubstitution is not featured on the Controlling Max with Messages page. Googling with, 'max msp definesubstitution' returns nothing of this option.

In my Packages/mypkg/init/mappings.txt file I have

max objectfile mytest mytest; max definesubstitution mytest bpatcher @name mytest.maxpat @offset -20 -20

but from the bpatcher reference this means this file format means I can specify attributes such as @name, but Common Box Attributes are ignored - I've tried adding @size 70 70 but the object does not render to those dimensions.