Sider is a companion program for PES 2021 and PES 2021 Lite. It allows for small tweaks - such as "free side-select" (which is where sider gets its name from), and time expander for the demo version. It also facilitates game mods, created by the modding community. The two main features are LiveCPK and the scripting engine, where you can write your own mods in Lua programming language.
LiveCPK makes it possible to replace game content at run-time with content from files stored on disk, instead of having to pack everything into CPK-archives. (This feature is similar to Kitserver's AFS2FS and to FileLoader for earler versions of PES). Depending on how and when exactly the game reads the replaced files, you can even live-edit some things, like textures or 3D models, without exiting the game.