Thank you for your quick answer; I should have found by myself the relevant PL documentation entry : thanks a lot and my bad !
I actually searched unsuccessfully docs & forums with the "additional" and/or "selector" keywords... whereas you / PL refer to custom and/or HTML tags. Please forgive me here for my school-level ant not fully W3C compliant English
In concrete terms, using these PL Typography section advanced options, I swimminly set up almost all font properties for some common logical tags (such as emphatic & strong emphatic) that is family, variant, weight, etc. Except style property : the HTML <em> tag, despite being set to normal with this feature, still displays italic text...
This considered (what works fine or does not), I'm still wondering what's the real interest of this feature : tips & tricks I missed ? benefits & drawbacks versus "global / whole site" custom CSS ?
Notwithstanding these questions, I suppose you may consider my problem has been resolved