This read-write TruthValue, settable only at entry creation and immutable afterward, indicates whether a session set is fault tolerant (true, allowing up to two groups) or not (false, allowing only one group). An admin cares because this decision at creation time permanently shapes how much redundancy that session set can offer. For example, an operator wanting MGC session redundancy would set this to true when first creating the session set, since it cannot be changed to add a second group later.