The .qcow2 extension is a QEMU copy-on-write disk format, the standard for Linux-based hypervisors like KVM, which powers both GNS3 and EVE-NG. Key Features of Version 15.9(3)M2

The adventerprisek9 designation signifies that it includes the "Advanced Enterprise Services" feature set, supporting MPLS, VPNs, and advanced IP routing protocols.

The vios-adventerprisek9 prefix indicates this is a Layer 3 router image.