ProFantasy Software produces expansions for CC called add-ons. They do not work without CC. Add-ons furnish you with new tools for a dedicated purpose. They give CC more mapping muscle.
It's best to use an example. Say you wanted to devise a town with just CC3+. You start with a blank map, add roads, then select buildings from a symbol library, and then place them. Add labels, and you have a pretty nifty town. Now, imagine you have City Designer 3 as well. You start with the same road layout, but you have a choice of 1500 symbols in an array of styles, which you can slide around your roads. You can create buildings of any shape or size with a few clicks or even create whole random streets on the fly. When you've labeled your maps, City Designer 3 can add an alphabetical street index with coordinates. Click on a street name, zoom to the street. And this is just a sample of what CD3 can do. Add-ons gives you more options, and lets you build your city faster.
The add-ons are separate from CC because we want to give you a choice - if you don't need card models, don't get Dioramas Pro; if you rarely create urban areas, you don't need CD3. If you have CC, and need to expand your campaign, you can get the appropriate add-on. All the add-ons benefit from the powerful features of CC's CAD engine, and you can use any add-on with any design.
Symbol sets are extensive collections of extra map-making styles and symbols to give you more cartographic choices. They make your maps more distinctive and attractive, at a modest price. They plug in seamlessly to CC, so you if you know how to use CC, you already know how to use them. They all use the features of add-ons where it's appropriate.