(this can be used for general discussion of per-thread data)
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[CMcC] Each of the following [tcl core subsystems] contains Thread Specific Data:

    * tclFileName
    * tclIO [Tcl Core IO System]
    * tclNotify
    * tclAsync
    * tclEvent
    * tclTimer
    * and even tclRegexp

It's used, somewhat like ClientData, to store per-thread quasi-global arbitrary data,
which is required to maintain state, and used to clean up on termination
or [Finalization] as it is called in the [tcl core]

This is useful, because it helps modularise each subsystem, however the very encapsulation
makes it impossible to extend or modify the subsystems additively, which would make it easier to
test subsystem mods without pulling apart entire subsystems.

I'm writing a TIP on this subject.