Since its used by the Stepoko folks who want to be pure opensource, there are some resources on it there:
and we have this post:
(linked to from: https://www.shapeoko.com/wiki/index.php/Universal-G-Code-Sender#Macros )
Hopefully that will cover it, if not, Will Winder has some documentation up at:
If macros aren’t covered, mention it to him?