This movie is using so called “stacked files”. The naming is a bit off from the official naming schema though.
Stacked files will play sequentially. You can use that if your movies come in multiple parts – e.g. the Lord of the Rings movies that were delivered on 2 discs.
See “Movies Split Across Multiple Files” in the support article on naming and organizing your movie media files.
Personally I prefer to actually append (merge) those files as not all Plex players can deal with stacked files. This can e.g. be done quite easily using MKVToolNix (even for non-MKV input files)
