Splats, which I’m defining as “sourcebooks that share the core game but can be their own game” for the purposes of this article, were popularized by White Wolf in their Chronicles of Darkness line (originally called “New World of Darkness”). It can be a great way to expand on your engine and game-line without bloating your main game.
The existence of each other does not increase the rules complexity of each other and expands the types of adventures the engine can have. People can still do mixed splat titles and intersect the rules if they so desire.
It isn’t a common tactic in traditional roleplaying games — normally, sourcebooks expand on the main game and its play experience–, but it is an interesting alternative.
Powered By the Apocalypse (PbtA) is one of my favorite engines and I love seeing how people play around with it. Some fans of the engine have tried working in splat design to the engine to emulate some of their favorite settings and its interesting to examine the mechanics behind it all.