Best Of

this did it:

export function getWorkspaceUnits(definition is map) returns map
{

/*
    In precondition area:
    
    annotation { "Name" : "Length Unit", "UIHint" : UIHint.ALWAYS_HIDDEN }
        isLength(definition.lengthUnit, {
            (inch) : [-10000, 1.0, 10000],
            (meter) : 1,
            (centimeter) : 1,
            (millimeter) : 1,
            (foot) : 1,
            (yard) : 1 } as LengthBoundSpec);

    Usage:
    
    const wsu = getWorkspaceUnits(definition);
    var unitlessLength = length / wsu.unit;
    outputString = toString(roundToPrecision(unitlessLength,3))~" "~wsu.unitText;

*/

    var resultUnit = definition.lengthUnit;
    var unit;
    var unitText;

    if(tolerantEquals(resultUnit, 0.0254 * meter)){ unit = inch;       unitText = "inch";} 
    if(tolerantEquals(resultUnit,      1 * meter)){ unit = meter;      unitText = "meter";} 
    if(tolerantEquals(resultUnit,   0.01 * meter)){ unit = centimeter; unitText = "centimeter";} 
    if(tolerantEquals(resultUnit,  0.001 * meter)){ unit = millimeter; unitText = "millimeter";}

    var workspaceUnits = {"unit" : unit, "unitText" : unitText};
    
    return workspaceUnits;

}

Custom feature execution can't know about workspace units - otherwise, we'd have to regenerate the entire thing any time you change those. The workspace units are basically just for display. If custom feature code determines a part name, it can't depend on workspace units.

You could have your part-naming feature take units and a precision in the UI, and round to that?

There is a way you can get the current workspace units at the time you insert the feature. I learned this trick from Kevin:

https://cad.onshape.com/documents/e0c13dc3836c7dcba155622a/w/c3a138bc21a0284d4afbea62/e/a2290d1b855aec5d8f5af6f3…

Get the current default units when the feature is inserted. This input will default to 1 of whichever units the document is currently using at the time. Note that it only does it when you first insert the feature.

annotation { "Name" : "Invisible length", "UIHint" : UIHint.ALWAYS_HIDDEN }
            isLength(definition.lengthUnit, {
                            (inch) : [-10000, 1.0, 10000],
                            (meter) : 1,
                            (centimeter) : 1,
                            (millimeter) : 1,
                            (foot) : 1,
                            (yard) : 1 } as LengthBoundSpec);

and then, convert and round. Now that I'm looking at this next part, the switch statement might be redundant since the lengthUnit is already a resulting unit.

//get document units
    var resultUnit = switch (lengthUnit) {
            (inch) : inch,
            (meter) : meter,
            (centimeter) : centimeter,
            (millimeter) : millimeter,
            (foot) : foot,
            (yard) : yard,
        };

    var unitlessResult = distance / resultUnit;

    if (rounding)
    {
        distance = round(unitlessResult, roundingPrecision) * resultUnit;
    }

@martin_kopplow

I hacked myself a solution for now, but it could be better. Maybe one of the good FS coders will help me…

You can now use MCs, Arcs, Circles, Curves, and Lines. and it names the final sweep with the length

https://cad.onshape.com/documents/6661cf93ab8de38ee21084ba/w/b8355f5cf6f02bcaeade2553/e/26b8cb50d10f4b14394f2077

It has been years and we still don't have foundational knowledge of how featurescript operates.
From what I can see in the forums most of us dev's have been wasting hours upon hours guessing and testing.
This post and others are "weak-sauce" at best. We all know about:
-"forEachEntity"
-"makeRobustQuery"
etc, etc and may have some idea on how to use them effectively but no solid understanding on exactly what's happening.
And please for the love of all that is good NO METAPHORS OR ANAOGIES! just tell us exactly what is happening.

If I have to decode one more post that starts with "Look at this way", "Imagine it like this", "It's sort of like", "I always picture it like this" then I'm lighting my PC on fire.
We're big boys we can handle the complexity without cryptic stories and riddles.