In the community, "APK Mod" typically refers to third-party modified versions of the Plex client or server designed to bypass the following restrictions:

A specialized version of the server backend, officially supported primarily for the NVIDIA SHIELD TV . Unlike the client app, the server APK is responsible for scanning libraries, fetching metadata, and transcoding video files. Sideloading and Compatibility

While PMS is officially intended for specialized hardware like the NVIDIA SHIELD, users frequently sideload the Plex Media Server APK onto standard Android phones and tablets to repurpose them as portable servers.