Blackberry data services over WiFi - Official Document

Hey guys, I found this while looking around:
www . cisco. com/en/US/solutions/collateral/ns340/ns394/ns348/ns772/networking_solution_solution_overview0900aecd806f4 3ca.pdf

I read it, but since I am very new to BB I have some questions..

My carrier charges me by a "Pay as you go" plan, so Whenever I use data it charges me.
If I have BIS activated, and in a place where there is Wifi, when I use my BB does it use wifi to get data, or use the carriers plan??