Both! First, you upload a mix as your main RECord so everyone can hear it by just hitting play on the site. Then, upload your individual tracks (the ones which make up that mix) as your working file; The working file is an option you’re given when uploading your RECord to the site which stays joined with the main mix file. Since this working file gives the community access to your individual tracks and layers, the working file is at the heart of how we collaborate on hitRECord. So always include your individual tracks in your uploads if they’re available to you. Without them, the chances of your music getting REmixed minimize drastically.
Individual tracks or stems should all begin at the same point in time (ideally, at the very beginning of the song / lead-in count), even if it means lots of 'dead air' in the file. This way, they can all be lined-up to play in sync correctly, and to be time-stretched appropriately as necessary.