Vs. Tests

Hello -
I’m reading through BWG and really liking the system. I had a lingering question regarding… well, a lot, but we’ll start with this. For the purposes of advancement, what is the Ob of a vs test? is it the number of dice the other person throws? or is it another number. I read carefully through the pertinent section, but I must be missing it. Please let me know where the answer is!

Number of successes your opponent got on their roll.

Edit to add: Sorry, bottom of page 45.

Thank you!