If you have ever looked into the art directory folders (symbols/fills) inside the CC3+ data directory, you may have noticed one thing, there appears to be four copies of every file. Why is that? And why is four files better than one file?
Well, the answer here comes down to quality and performance.
The main issue that occurs when computers need to display an image on the screen is that the image must be scaled to fit the place it is being displayed. And this is not a free operation for a computer, it is actually a bit of a resource-intensive one. If you look at any random overland map, like the example map from the manual shown here, you’ll see that it is full of symbols. But all those symbols are pretty tiny. When zoomed out so you can see the whole map on screen, even in full screen, each of those trees are only something like 25 by 25 pixels on screen. Obviously, the symbol itself is much bigger, because it is not supposed to just look good when zoomed out like this, but also when you zoom in closer. And that means that for every one of those trees, CC3+ will have to take a much larger image, load it in to memory, and then resize it there before putting it on screen. A modern computer can do that pretty fast, but a map doesn’t just have one symbol, it can have thousands of them. And this process needs to be done anew each time you change the map view, like scrolling or zooming.
And that is where having four files comes into play. Each of these files are a different resolution. which means that instead of having to resize that large high-quality symbol each time, we can instead start with a smaller file that is closer in size to the size we want to display it in. This allows for a much faster operation as they are a smaller file size, making them faster to load from disk, take up less space in memory, and are faster to resize as well.
And, it also allows us to provide our artwork in a very high quality. CC3+ offers most of our art in a much higher resolution than most other mapping software out there. If we only provided the artwork in that very high resolution, it would affect program performance, but thanks to the four different resolutions available, those high-quality files are only used when actually needed, such as when zooming in really close, otherwise the faster smaller files are used. This results in better performance while still supplying really high resolution artwork.
If you look at the naming of the files, they all have a set of common suffixes, _VH (Very High), _HI (High), _LO (Low), _VL (Very Low). The exact resolution of these actually depends on the type of map, this is because floorplan maps are typically intended to be viewed at a much higher detail than an overland map.
The sizes are all given in pixels per map unit. As you may know, map units are an abstract unit that varies with map types, for example for overland maps map units are miles/km, while for city and dungeon maps they are feet/meters. Other map types have their own definition based on the sizes they depict. For overland maps, the highest resolution symbols are normally provided in 20 pixels/map units, so a feature intended to be 5 miles long would have a symbol 100 pixels long. For city maps, the size is 40 pixels/map unit, so a house 20 feet long would be an 800 pixel image, and finally, for dungeon style map, the typical size is 100 pixels/map unit, meaning that the 1 foot long dagger would have a symbol 100 pixels long. These sizes are all for the highest quality (VH) version of the artwork, the HI version is 40% of the VH version (so if VH is 1000×1000 pixels, HI is 400×400), the LO version is 10% and the VL is 2%.
Normally, you would only need to worry about the highest quality version if you make your own symbols, as CC3+ will automatically make the lower resolution versions for you when you import your symbols using the Import pngs command from the symbols menu or the Import bitmap fill styles from the tool menu, as long as you tell it to.
If you need to, you can also tell CC3+ exactly which resolution to display. If you go to the Display Speed Settings 
dialog you can either set it to Automatic, in which CC3+ will decide which file to use (reduce the quality for faster redraws but lower quality), or you can set it to Fixed bitmap quality, in which case you can tell CC3+ exactly which variant to use, Very High, High, Low and Very Low, matching the four image variants.
Note that for normal work, automatic is strongly recommended. You’re not really gaining anything in quality by forcing it to very high, it is mostly just a troubleshooting tool if things don’t seem to render as they are supposed to. All that will happen is that redraws will be slower. This also applies to exports. When you export a map, if your export resolution is sufficiently large, CC3+ will use the Very High version of the artwork during export even if that is not the one currently used for displaying the map on the screen. Reducing the quality can make redraws be faster while you work on the map, but if you do that, do remember to set it back to Automatic high before exporting.
Finally, do note that CC3+ can also work with fewer than 4 versions. This is typical if you acquire third-party artwork that comes in a lower resolution than what is typically offered with CC3+ VH files. Often such files are more closely matched to the HI version in CC3+. In this case you can tell CC3+ that your source files are simply “High” instead of “Very High” in the import dialog, and CC3 will happily just create the two lower resolution and work with HI being the best available for that particular symbol or fill. This ensures that it work properly along the other symbols and have the right resolution version picked, except when VH symbols is called for, when such a symbol would simply just display in the highest available.
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