Only one radio button in the group can be on at a time; when the user clicks a button to turn it on, the button that's currently on is turned off. One button in the group must be on at all times; the user can turn a button off only by turning another one on. The button that's on has a value of 1 (B_CONTROL_ON); the others have a value of 0 (B_CONTROL_OFF).
The BRadioButton class handles the interaction between radio buttons in the following way: A direct user action can only turn on a radio button, not turn it off. However, when the user turns a button on, the BRadioButton object turns off all sibling BRadioButtons - that is, all BRadioButtons that have the same parent as the one that was turned on.
This means that a parent view should have no more than one group of radio buttons among its children. Each set of radio buttons should be assigned a separate parent - perhaps an empty BView that simply contains the radio uttons and does no drawing of its own.