Pop quiz: what zoom level does that statement represent?
How about
Same thing, zoom 12, but the answer is obvious for the second statement.
Zoom level specification statements represent about 1/4 of all statements in osm.xml. Here is how to make those statements self documenting.
I saw this table relating Scale to Zoomlevel, abridged here.

Zoomlevel Scale

XX Sorry that table didn’t come through at all, but here’s the gist

Here we can see ([zoom12 scale]136945 + [zoom11 scale]272989)/2 = 204967 and that most likely some human looked at that number and said “200000 is close enough”.
I don’t see any obvious way a -z12 specification in a .mss file gets translated to the 200000 part of ScaleDenominator> statement so I expect it is hard coded into the carto program.
What I suggest here is that the lower two digits of a number defining a zoom level be set to the zoom level the value represents. For example the zoom level progression would change as :[current 750]->720, :[current 1500]->1519, :[current 2500]->2518, etc.
Someplace there is a method that translates ‘z12’ into 200000. It may be as simple as a table. If the method could be altered to return the self documented ScaleDenominators then 1/4 of the lines in osm.xml will be instantly documented and more readable.
-RickBrown in Reno, NV

What software are you talking about ???

Talking about making osm map tiles with mapnik. Specifically, the osm.xml file. Sorry for any confusion