Does anyone know what this valve is? I know it's partly an isolation valve, but am a bit puzzled by the slanted hexagonal part. I saw it under a basin. Is it maybe a combined pressure reducer?
When the MIs specify. Generally before something that would be damaged by grit or scale flakes getting through. Water softeners, TMVs etc etc normally have them spec'd.
One of the items on the "low flow rate complaint" checklist (along with prostate disease) is filters like this which have become clogged, doing their job, and need a clean.