> Does anyone has any idea about what "copy_rule" of Signatures.PLUGIN does?

IIRC [copy src dst]  copies the file or pattern [src] from your source
tree (and maybe also from _build if it exists there but I'm not sure
anymore) to [dst] in the _build directory.

> it says "copy_rule name ?insert source destination" in the documentation,
> is "name" here the name of the rule in "source" file?

It is the name of the rule, the one that will be printed by ocamlbuild
-documention.

Best,

Daniel