CSR stands for Cross-Server Relay, and is a plugin in development aimed to replicate a player character's motions and actions across to one or more other servers using bots.
The target use case is to allow speedrun participants from around the world to race each other as if they were all on one server, but in reality they are merely connected to the server closest to them. This means they can all race with decent ping, rather than the unfortunate scenario of someone on a US server having 30 ping racing someone from Asia with 300.
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Progress update about CSR:
The plugin is being rewritten from scratch using the new Sourcemod 1.7 syntax. It already resolves 1 of 2 issues that prevented the release at TotH.