Purpose With RadioButton you can create radiobuttons that correspond to the value of a variable. All radiobuttons with the same variable name are considered to be a group, i.e. when the user clicks on one of them, the others are turned off. 

Please note: In addition, there is a RadioButton syntax for replacing an input field with a series of radiobuttons. See GuiXT Documentation Radionbutton. 
Example // Set default office: Boston
if not V[SelOffice]
  Set V[SelOffice] "60"
endif

// Office selection
Offset (8,0)
Box (0,0) (4,30) "Select Office"
RadioButton (1,1) "Boston" Name="SelOffice" Value="60"
RadioButton (2,1) "Paris"  Name="SelOffice" Value="62"
RadioButton (3,1) "Roma"   Name="SelOffice" Value="80"

This defines 3 radiobuttons. The internal variable  V[SelOffice] is set to one of the values "60", "62", "80", according to the selected radiobutton.

Format RadioButton (row,col) "text"   Name="..."  Value="..."
Options
-readOnly Input not possible
Tips & Tricks See InputAssistant tutorials 3 Parameters and inputfields and 4 Fields on tab controls.
Components GuiXT + InputAssistant