What do you mean by reference? As far as I'm aware there isn't a single reference, you have to piece things together from different sets of documentation.
In addition to the links Thomas has provided I would look at https://developer.atlassian.com/display/CONFDEV/Confluence+Objects+Accessible+From+Velocity and the API at http://docs.atlassian.com/atlassian-confluence/latest/ to see what objects and methods are available for you to use in your user macros.
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