Pricing when used with aws waf

Hi, Want to understand how pricing is calculated for F5 WAF rules. when we say price is 

Charge per month in each available region (pro-rated by the hour)$20 / unit
Charge per million requests in each available region$1.2 / unit

What unit represent here ?

When used with AWS WAF, would it be in addition to what aws is already charging based on their capacity units ?

Thanks