AP3 file format is a VS10XX VSOS App from Asprova – an APS (Advanced Planning and Scheduling) system for Microsoft Windows OS. In short, it saves the scheduling project created via Asprova. Naturally, other apps can use files with .ap3 extensions. Just be cautious with AP3 data from unknown sources.
MKV is a container format for holding multiple audio, video, picture, and text data such as subtitle tracks. It is similar to other common containers like MP4 and AVI, although MKV is open in the specification. For video contents, MKV uses a .mkv filename extension, .mka for audios, and .mks for subtitles.
MPC-HC, KMPlayer VLC Media Player MKV File Player MPV