Slotting is how well-centered on pitch notes are when you play them. A horn that slots well requires very little, if any, compensation to play in tune (assuming the player is up to the task!). You play a note and WHAM! - the note is right-on, dead center on pitch and timbre. Some prefer a loose-slotting horn for jazz, so notes are easy to bend. I mainly play music that would fall into the "classical" category, but when I need to, I don't have any trouble bending notes on a tight-slotting horn. It may have something to do with using a larger mouthpiece. I prefer tight-slotting horns because I can largely forget about intonation issues and concentrate on other aspects of making music.