GroupBox – PB Docs 2018

You use a GroupBox to group a set of related controls. When a
user tabs from another control to a GroupBox, or selects a GroupBox,
the first control in the GroupBox gets focus. To tab between controls
in a GroupBox, set the tab value of the GroupBox to 0 and assign a tab
value to each control within it.

All RadioButtons in a GroupBox are considered to be in a group.
For more information about using RadioButtons in GroupBoxes, see RadioButton.