So here's my quick answer every body gets. It's a global treasure hunt, everywhere except North Korea for obvious reasons, and any dangerous places you don't want to send people like train tunnels. We don't do near elementary schools ether for fear of being thought of as sandbox uncles lol.
The difficulty of the hide and the size of the hide very, as well as the creativity. Also it's up to the cache owner to maintain and keep the hide fun.
Trackables are bugs that travel from cache to cache via geochers traveling all over the world. Some are only discoverable like me, some you take with you and place in another cache. Many airports have travel bug hotels with people picking up and dropping off as they come and go.
Often the cache is just a log, to sign, it's the geographic location that may have historical significance, or it's just a hidden cool place somebody wants to show you.
Some times category 5 difficulty require diving experience, or climbing. Some times you have to go at low tides, into water caves. Sometimes there puzzles to solve. Multi stage caches.
They can be the size of your pinky, or a trap door to a room of treasure.
Always read the logs of past finders, and see if it had fav points. There's a lot of bunk caches out there that's for sure..