Guides and Metrics for smcp

How are the visual guides in the Glyph Edit window for things like CapHeight.smcp and Baseline.smcp controlled / set / turned on and off?

There is no reference to this in the User Guide or this forum that I can find …

They are shown only if H.smcp (or h.smcp) and x.smcp exist.

They are shown only if H.smcp (or h.smcp) and x.smcp exist.

… and I’m guessing they take their metrics from those glyphs … Thanks!