it looks like a legacy command for Carrier Grade NAT not present in the documentation, but it should enable a particular NAT mode.
this command drops all packets which can not be Port Address Translated (non pattable trafffic). PAT can only be performed
on protocols where the ports are known : UDP, TCP, ICMP
If the router receives a non pattable packet, a static translation entry is created (1:1) and therefore a pool exhaustion is likely. Either make sure that only the above mentioned protocols hit the router or enable this command to avoid pool exhaustion. The documentation explains that pretty well:
When non pattable traffic (not ICMP, TCP or UDP) pass the gateway, it creates a reverse path to the inside host by installing a static entry in the NAT table. It can be seen as a backdoor because traffic can go outside to inside through this translation.This is a known issue :
The command "ip nat settings nonpatdrop" has been introduced in IOS 15.5(3)S4 (IOS-XE 3.16.4S) to fix this issue :
See bug CSCvd85915