I won't repeat the excellent definitions already provided, but can share some examples of class parameters where we use them:

Parameter FILES = "/files";
Parameter INSTANCE As COSEXPRESSION = "$p($SYSTEM, "":"", 2)";
Parameter RUNLEVEL As COSEXPRESSION = "$li($lb(""dev"",""Test"",""LIVE""), $lf($lb(""HCDEV"",""HCTEST"",""HCLIVE""), ..#INSTANCE))";

We use ..#FILES to prefix or normalize a directory off the top-level filesystem on the server.

We use ..#INSTANCE if we need to identify which IRIS instance, like in an email message, it came from.

We use ..#RUNLEVEL in conditionals or pass as a parameter, where we might want code only to run on "Test" or "LIVE".

lol, reading is fundamental.  Thank you for pointing out what should have been obvious.

I missed the "User" in your example as a custom implementation.  I just reviewed these provided adapters, very nice!  What continues to puzzle me is why the product does not provide a Business Operation off them (and others).  I know, support issues, right?

I just completed a custom "adaptation" using their provided Util.Pipe adapter, because the code was a bit crusty.  It did serve as nice reference source, just not as nice as these SQL adapters.

... because we're running 2018 and that SQL operation does not exist to use in our operations:

Cache for UNIX (Red Hat Enterprise Linux for x86-64) 2018.1.4 (Build 505_1U) Thu May 28 2020 10:11:16 EDT [HealthShare Modules:Core:15.032.9035 + Linkage Engine:15.032.9035]