This entity is the color palette the drawing is using. A custom color palette is one of the hidden entities that are possible in a map. You could detach it from the drawing presets dialog, but this will have your map revert to using the default palette.

@ is the standard CC3+ reference to the CC3+ data directory, the default location unless you changed it during installation is c:\ProgramData\ProFantasy\CC3Plus

System is a folder within the data directory.

You'll find a lot of it in the tome somewhere, but there is no dedicated section just describing the special sheets, it's more a thing you pick up a bit from here and a bit from there.

For the SCREEN sheet, it's not that special, really, but because the canvas in CC3+ is unlimited in size, if you put a symbol at the "edge" of the the map, it won't be clipped, it will simply extend off it. The SCREEN sheet typically holds a white polygon that simply covers up all those bits sticking out.

For the MAP BORDER, the map border sheet just holds the graphical decorative map border. It is usually placed last in the list to put a nice frame on top of things. The "technical" map border, which define the size of the maps, and that drawing tools use as the edge to know where to stop drawing, is simply 4 lines on the MAP BORDER layer, they can be on any sheet, but usually the live on BACKGROUND.

Basically yes. You can consider sheets a feature for visual grouping/ordering (+ effects), while layers are logical grouping.

Layers only end up important if you actually use them though. Having everything grouped according to feature, such as one layer for hills/mountain is nice and all, but if you never use the features layers provide (select by layer, hide layer, freeze layer) it becomes less important. I do find it useful to be able to select things like all the mountains though, for things like sorting symbols. While the layers doesn't themselves determine what goes on top of what, the ability they give you to select just the correct symbols is nice.

Layers are actually more important for the interlaces of the software than sheets though. For example, dungeon corridor tools rely on layers to figure out what is a floor and what is a wall when connecting to existing parts, cutting symbols rely on layers to align to and cut that wall when inserting a cutting door, and the edit feature of drawing tools relies on the layer to determine if this is an appropriate entity to edit, the background of the map is figured out from the BACKGROUND layer, and the extent of the map is determined by entities on the MAP BORDER layer to name some.

Usually though, tools set their own layer when you use them, so you don't really need to keep to much an eye on that yourself. If you create your own tools and symbols, you should always set up layer information so your own tools can also set layers automatically, but unless actively use your layers manually, it is usually just fine to then let the tools do their thing, but don't otherwise think too much about it.

Technically, yes. But normally, you'd find at least BACKGROUND, SYMBOLS, GRID, TEXT, SCREEN and MAP BORDER in all maps, as they are commonly used. Additionally, for a dungeon map, you would normally have at least FLOORS and WALLS.

Which sheets to include does depend a lot on the tools and symbols you want to use. If you make everything from scratch, you are pretty free since you then also configure this yourself on your tools and symbols, but if you want to be able to use tools and symbols from other styles, you should look at what they expect.