Finally got it.
I was travelling and only reading on a mobile phone, but now I've seen, that this only refers to the lines file of the mkgmap default style. I was under the assumption, that this might have been some mkgmap inherent (built in) functionality. 
Sorry.

From: Felix Herwegh [mailto:mlmmduk@herwegh.de]
Sent: Wednesday, March 6, 2024 at 5:16 PM
To: Development list for mkgmap
Subject: [mkgmap-dev] Commit r4918: add surface=chipseal to group of surfaces which means paved

Ahh, thanks. 

ATM I'm putting in some effort, to select and flag (sufficiently) paved surfaces manually in my own style.

Is this group just a selection of surface tags? Are the inner workings documented somewhere (except for inthe code)?


-------- Original Message --------
From: "Carlos Dávila" <carlos@alternativaslibres.org>
Sent: March 6, 2024 2:41:02 PM GMT+01:00
To: mkgmap-dev@lists.mkgmap.org.uk
Subject: Re: [mkgmap-dev] Commit r4918: add surface=chipseal to group of surfaces which means paved

This group sets mkgmap:unpaved=0 for all surface types included

El 6/3/24 a las 13:22, Felix Herwegh escribió:
group of surfaces which means paved
What would be the role of this group / how could this group be used?

Auto-setting mkgmap:unpaved?

Thanks  Felix


-------- Original Message --------
From: svn commit <svn@mkgmap.org.uk>
Sent: March 6, 2024 8:03:28 AM GMT+01:00
To: mkgmap-svn@lists.mkgmap.org.uk, mkgmap-dev@lists.mkgmap.org.uk
Subject: [mkgmap-dev] Commit r4918: add surface=chipseal to group of surfaces which means paved

Version mkgmap-r4918 was committed by gerd on Wed, 06 Mar 2024

add surface=chipseal to group of surfaces which means paved
Patch chipseal.diff by Carlos Dávila


http://www.mkgmap.org.uk/websvn/revision.php?repname=mkgmap&rev=4918
_______________________________________________
mkgmap-dev mailing list
mkgmap-dev@lists.mkgmap.org.uk
https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
_______________________________________________
mkgmap-dev mailing list
mkgmap-dev@lists.mkgmap.org.uk
https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
_______________________________________________
mkgmap-dev mailing list
mkgmap-dev@lists.mkgmap.org.uk
https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev
_______________________________________________
mkgmap-dev mailing list
mkgmap-dev@lists.mkgmap.org.uk
https://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev