The string you mentioned looks like a specific filename for a used in networking, particularly for Cisco routers. Breakdown of the Filename
The name encodes technical details about the software's architecture, feature set, and release date: i86bilinuxl3adventerprisek9m21573may2018bin
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 string you mentioned looks like a specific
: This is the standard file extension for a binary executable image. Significance in Network Engineering i86bilinuxl3adventerprisek9m21573may2018bin