Pressing the D button cycles through several different numerical display modes, designed to help you get a better picture of what's being represented in the battlescape display. If you have difficulty seeing things, remember to disable fog of war (press F) and lighting effects (press L) for better visability. You can also cut or paste over tiles if they're in your way.
1. Smoke/Fire Duration / 2. Smoke/Fire Effect
The first two modes deal with smoke and fire. A single press of the button will convert all patches to numbers representing how many turns they'll last before dissolving. A second push will then show you how many tiles worth of visibility they obscure. The editor allows you to tweak these values.
3. Route Nodes
Reveals all the route nodes on the map. These define the points aliens patrol between when not actively hunting your units, and also control where units initially appear on the map. You can't edit the nodes themselves, but it's possible to use the unit editor to point aliens towards certain map areas.
4. Lighting Effects All tiles will include a value showing their current light level. This is determined from the lowest value that SourceMP.Dat or TerMP.Dat has for that map location.
5. Tile Visability Shows the SeeMap.Dat entries for each tile. In a nutshell, if the value is less then four then the tile will be hidden by fog of war.
The nodes in the bridge of a downed Floater Battleship.