@Niara The CR system is messy when it comes to certain things. CR is in reference to how it as a enemy can challenge a group of adventures. Depending on the level range of the campaign you have to account for things as a DM like which items you reward to your players and team composition for example. Like a bandit at cr3 can have more hp and multi attack than a fighter at level 5.
For a campaign you don't have to adjust anything and could simply pick an enemy from a higher challenge rating or you can severely limit what rewards a party can attain or you can make some changes. Regardless an enemy from the campaign can, but does not have to, follow the same rules to the letter as players.
Their are all sorts of supplement books out there to help make campaign's. Some players, if you let them, will stomp an encounter into dust if you aren't careful. Some challenge without overwhelming the players helps.
