Constant MAX_FCNT_GAP 

pub const MAX_FCNT_GAP: u32 = 16_384; // 16_384u32

How far ahead of the last counter it accepted a receiver follows a sender.

RP002-1.0.5 lists it for LoRaWAN 1.0.3 and earlier and notes it was removed from 1.0.4. A frame further ahead than this is refused, which is what stops a captured frame being replayed at a counter the sender has not reached yet.