an incoming packet on that specific interface sources from an address that does not exist in the routing table or exists but with an exit path toward another interface against the one on which it arrives
In your case you have either "ip verify reverse-path interface outside" and "ip verify reverse-path interface Inside" feature enabled on both outside and inside interfaces, so what happens is when an incoming packet arrives at the outside and/or inside interface it would be subject to the source validity check (uRPF), if a route exists for that packet flow it would be passed through, otherwise if a route does not exist, or exist but not match the flow, the packet would be denied.
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