SLATE_BEGIN_ARGS and SLATE_END_ARGS actually create a struct containing variables added by the SLATE_ARGUMENT, SLATE_ATTRIBUTE, and SLATE_EVENT macros.
The name given to those variables is in the form of _TheNameYouGaveIt.
SLATE_ARGUMENT creates a variable of the exact type you gave it, so SLATE_ARGUMENT(int, MyThing) makes a variable of type int, with the name _MyThing.
SLATE_ATTRIBUTE creates a TAttribute for the type you gave it, so SLATE_ATTRIBUTE(int, MyThing) makes a variable of type TAttribute, with the name _MyThing.
Additionally the macros define a series of functions to allow you to set the variable they add, which are named based on TheNameYouGaveIt.