{??} is a query string macro. Take a look here at the types of macros available.

In regards to the other web part, is it one you created or an out of the box one?

If you do need to get the Username and it's not available in the standard kentico macros. You can create your own custom macros which return the information you need.

You can also do custom methods and functions within your transformations, see here:

http://devnet.kentico.com/docs/devguide/index.html?adding_custom_functions_to_transformations.htm

This is really useful at certain times when the standard macros don't do exactly what you need.

If you need anymore info let me know,

Thanks,

Matt