In addition to the int, float, double and UINT datatypes, there is another option in Plugins->Add/Remove User Variables that is named:

VSTGUI

This is a special type of variable that is actually a float datatype with preset unipolar normalized limits such that it can vary between 0.0 and 1.0. The GUI controls in VSTGUI4 which RackAFX uses can all be linked to normalized variables. Adding User Variables of this type allows you to declare GUI-linked controls for the custom VSTGUI only - they do not appear on the RackAFX UI. This will allow you greater flexibility in how you use your VSTGUI controls since you are no longer tied to the RackAFX UI. Note that this is an advanced feature and if you are still a RackAFX novice, you should stick with the normal GUI operations. But for power users, this will allow you to come up with some very interesting GUIs.

When you open the GUI Designer, you will see all of these VSTGUI variables in the Control Tag lists and you can then connect the GUI elements to them.