Right, there are ten things to choose from there. The idea is, you want to give out each of the ten parcels per level that the characters advance.
So, beat a small, non-story side encounter? Might have no treasure at all, or might get parcel #8, 9, or 10. Beat the bosses' lieutenant? Maybe parcel #2, 3, or 4 plus parcel #7, 8, or 9. Beat the main boss? Parcel #1, maybe #4 or #5, plus #6 or #7 and maybe any others that haven't been awarded yet.
One neat trick is that you can "borrow" against future levels; for example, with a party of 5, you only get 4 parcels with magic items at Level 1, so one of the characters comes out "short", which is a little lame. No biggie. You can "borrow", say, parcel #3 from Level 2, to give everybody a magic item by the time they level up.
This is particularly cool if you vary things a little bit: maybe you know the party are getting into some fast-moving plot stuff, so as they battle through Level 12 they only get about half of L-12's parcels .. then in L-13 they fight a dragon. The dragon's hoard might include the remaining L-12 parcels, all of L-13's parcels, and possibly even an item or two out of L-14's parcels, for one truly epic haul of treasure.
By the end of L-14, though, you've given out all of the L-12, L-13, and L-14 parcels, so it all balances out in the end.