LibTmux.TmuxCommand.RequiredGeneration

A tmux ID such as %2 is only meaningful on the server that issued it; a restarted server reuses those IDs for different objects. A command built from an entity therefore records which server the entity was read from, and ExecuteAsync refuses to run it against a different one.

Null means the command names no entity -- a raw command, or one whose target is a name rather than an ID -- and carries no such requirement.