This is a read-only bitmask indicating which SNMP notifications, listed in nbsTrapListTable, are capable of being sent for this particular interface; bit 0 is reserved, and subsequent bits correspond in order to table entries, counting the MSB as bit 0. Admins consult this to know which traps are even possible on a given interface before trying to enable them via nbsTrapIfTrapsSelect, since not every trap type applies to every interface. For example, an admin might find that a virtual interface's capability bitmask has the bit for linkDown cleared, meaning that trap simply isn't supported there regardless of configuration.