What is created initially depends on what tools you use. If you use the basic tools (path, poly, etc) in the right-hand toolbars, they only create a single entity, either a line or a polygon, depending on the tool (Of course, if you draw using a polygon tool, but has line width greater than 0, it will appear as a hollow shape with an outline, looking more like a line than a poly, but it is still a polygon)

If you draw something using one of the drawing tools (usually accessed from the buttons in the top right, or the symbol catalog window) it may or may not draw two entities, depends completely on the tool. Some only draw a wall, some only draw a floor, and some draw both.

If you use the dungeon room/corridor tool, it will always make two entities, wall and floor.

In the cases where the tool created two entities, these are separate, and needs to be edited separately.

You can usually easily identify if something is one or two entities by using the List command from the Info menu. If it goes straight form 0 selected entities to 2 selected entities when you click on the edge, you know you are dealing with overlapping entities (probably a floor and a wall), and the info you get when executing the command should give the details on the entities.

When you use break on a polygon, it turns into a path (a line), which means it is no longer a polygon, and obviously therefore have no surface area to fill anymore, which is why the fill disappears.

Use the trim tools, such as :CC2BREAK: to create breaks in lines. Combine it with the various modifiers, such as :CC2MODINT: to start/stop breaks exactly where the lines meet with the line from the other room.

You can then combine the line segments into one path using tools such as Combine Paths if you need to. (If you need to depends on how you are going to use them. Just having individual paths that just meet up in the exact same point is often enough.)

The reason you couldn't find them is because they don't have CA51 in the file name. If you look in Sue's screenshot above, you'll see that the 'Dunes CA51' fill references a bitmap called 'Dunes.png'

That's why you can find them using that dialog, but you need to do so from a map in the appropriate style. From there you can find their names, and then in the map you want them in, you can then easily add them once you have learned that filename.

Not much. Mainly a new short chapter describing the Symbols along, fill with symbols, and symbols in area.

Try a fill with a more visible texture. It gets a bit harder to find something nice due to the perspective nature of the map, but you could try with something like the Stone Lava Cooled bitmap from DD3.

Additionally, the pixelation of your current fill is really bad, causing it to appear more like a uniform mess. Try decreasing the scaling of the fill as well. That leads to more repetition when zoomed out of course, so you may need to consider working a bit with the surface, applying layers of different fills and blending them using effects to avoid the repetition.