This boost completes practice of every single song in your reperoire (if they're 'checked' to practice of course) No matter how many are them. You can jam 10.000 songs if you're patient (or mad) enough to add that number of song to your repertiore.
If you choose extra sq, boost will divide 40% sq with every bandmember, that means you'll get extra 10% if you have three bandmembers along with you. That option is useful if you're solo, have more than 10% sq and want to have a stage name (you can get your stagename if your sq is more than 50%)
Ambition level will have an effect for your next show only. You'll show unusually good performance; that option is useful if you'll play for something really important (like competitions)