In an SDDC, availability zones are collections of infrastructure components. Regions support disaster recovery solutions and allow you to place workloads closer to your customers. Typically multiple availability zones form a single region. 

This VMware Validated Design uses two regions, but uses only one availability zone in each region. The following diagram shows how the design could be expanded to include multiple availability zones. 

Availability Zones and Regions
Region A, SFO, has two availability zones. Region B, LAX, has three availability zones.