Technically, they don't represent anything other than layers, and Base being the ones Sony gives you. However, mapfiend, generally uses layer 1 for map lines, layer 2 for labels, and layer 3 for grid lines (though it's rare to have grid lines).
Personally, I try to put zone labels on Layer 1 as well as the general map lines. I use layer 3 for more map lines (generally for vertical zones that are better split into 2 or 3 sections). I also often use Layer 3, for notes. And I use 1 and 3 for other miscellaneous, but important labels, that should stand out from the mass of labels in layer 2 .. this generally include traps and other hazards on layer 1, and tradeskill fixtures, bankers, etc. on layer 3.
Finally, I try to keep in-zone portal labels on whichever layer has the map lines for that portal.
Generally, I leave all layers showing, and just turn off labels. I often turn off Base or Layer 1 for zones with lots of duplication (PoK), and Layer 2 for when I want to see the important labels, but not the general mob / npc ones ... Sunderock Springs is an execellent example of having tons of general labels on layer 2.
Height Filter is great for zones with elevations on teh map lines, and lots of floors / levels. Unfortunately, many of the old world vertical zones don't have this information, newer zones though, like SHIP height filter is wonderful for. Generally, however, I have it off.
Filenames for the maps represent the layers:
zone.txt = Base
zone_1.txt = Layer 1
zone_2.txt = Layer 2
zone_3.txt = Layer 3
For specific information on the map file contents refer to
this thread.
Yther Ore.
Edited, Feb 18th 2009 4:40pm by Yther