Architecting a vCloud Director Solution : vCloud Director Design : 7.1 Provider Virtual Data Centers : 7.1.2 Tiers
   
7.1.2 Tiers
A single provider virtual data center can provide various tiers of storage represented by storage policies, but only one tier of compute. The provider VDC can span multiple vSphere clusters (resource pools), however it is bound to a single vCenter Server.
Multiple provider VDCs are created for the following reasons:
• The cloud requires more compute capacity than a single vCenter Server can provide.
• Tiered compute is required. Each provider VDC maps to vSphere clusters with different characteristics (performance or SLA).
• Requirement for workloads to run on a physically separate infrastructure (availability zones).