In following doc:
https://developer.apple.com/library/archive/documentation/CoreBluetooth/Reference/AppleNotificationCenterServiceSpecification/Appendix/Appendix.html#//apple_ref/doc/uid/TP40013460-CH3-SW4
eventflag 0x20 is reserved:
EventFlags
Table 3-3
EventFlagsEventFlagSilent = (1 0),
EventFlagImportant = (1 1),
EventFlagPreExisting = (1 2),
EventFlagPositiveAction = (1 3),
EventFlagNegativeAction = (1 4),
Reserved EventFlags = (1 5)–(1 7)
But spec has not been updated in this document since 2014. So the flag may now be used.
Note: found an old document (not dated, in chinese), where flags over 0x02 were reserved:
https ://www.programmersought. com/article/51571699993/
https ://www.cnblogs. com/alexcai/p/4321514.html#_EventFlags
which illustrates how reserved flags are progressively used.