Referral program. What does $25 billing mean?

I understand that on signup the referred get $10 and on completion of $25 usage as a DO customer by the referred, the referee gets $25.

It is here that I have a doubt.

Does the $25 billed amount include the original signup amount of $10 i.e. User gets $10 as signup credit and then spends $15 on top of it (total $25) to make the referee eligible for $25 in referral credits

Or is the $25 spend exclusive to the signup credit DO offers to the referred. So that the referred needs to spend $10 + $25 (own money) - (total $35) to make the referee eligible for $25 in referral credits

I hope an official statement from DO would clear the air better for all