Sounds like this is knit seamlessly to the underarm which will be the 10 sts you BO. The sections will be knit back and forth separately. At the armhole edge you continue to BO 3 sts at the beg of a row, then dec a st on 3 rows; this is usually done on every other row, rather than every row, though you have to do it at both edges of the back. At the neck edge (this will be the front sections, not the back section), dec 1 st every other row 13 times. To shape the neck on the back section, BO 15 sts in the middle for the nec, and BO 3 sts on both sides of that, then dec at each side of the neck 3 times.
It appears to be translated and can be confusing because they use BO, dec and knit together (which is the dec) interchangeably. Instead of the BO 3 times, you could k3tog instead though.
sue- knitting heretic