Some of the emulator media images in the media folder are in compressed format (ZST) to keep github file sizes below limits.

Files of type .dsk are disk images, .tap are magnetic tape images, and .ptap are paper tape images.

File names for disk and tape images start with two character names matching the devices they attach to. For example rk is is used for the RK05 disk drives with rk1.dsk attaching to RK05 disk drive unit 1. Similarly rp4.dsk would attach to unit 4 of the RP04/RP06 disk controller.

.DSK  disk images
.TAP  magnetic tape images
.PTAP paper tape images

The emulator main page lists what PDP 11 operating system is loaded on each of the available media files.