Brocade Extended Fabric VC_RDY R_RDY Buffer Flow Control

Hello,

As I understood from the Network Topology Guide, there are two ways for Buffer Flow Control: VC_RDY and R_RDY.
When should I use VC_RDY Buffer flow Control (means buying Brocade Extended Fabric License), and when should I use R_RDY Buffer Flow Control?
When I use R_RDY how can I configure the buffers?

Thanks in advance.

Best regards
Manfred