You won't find this file on a standard Cisco router. Instead, it is used in:
This is a Layer 3 image. It behaves like a router, supporting advanced routing protocols (OSPF, BGP, EIGRP) and various WAN features. i86bilinuxl3adventerprisek9m21573may2018bin
Most users upload this file to /opt/unetlab/addons/iol/bin/ . After fixing permissions, it becomes a selectable node in the lab interface. You won't find this file on a standard Cisco router
This specific string, , is a filename for a Cisco IOS software image. Specifically, it is a Cisco IOL (IOS on Linux) binary used primarily in network simulation environments like EVE-NG , GNS3 , or PNETLab . Most users upload this file to /opt/unetlab/addons/iol/bin/
The May 2018 build is widely regarded in the labbing community as one of the most stable releases. It suffers from fewer "memory leak" or "CPU spike" issues than older 15.x images.
The build or release date of this specific binary (May 2018).