All Cloud services run on some sort of hardware and they can categorized based on IAAS, PAAS, SAAS etc. But there can be one more way to categorize these service based on the availability, and these are Zonal, Regional, Multi Regional and Global.
As I am more into Google Cloud, let me define each one of the above with regards to the services provided by Google Cloud Platform.
Zonal Services
These services can be hosted or are available only in an availability zone (data center/server farm in a location) and if incase there is a failure or downtime in that availability zone, the service is no longer available.
Best example would be the compute engine service and the VMs that are spun up are located in an availability zone. This is true for all cloud providers where the VMs are available only in a particular zone.
So, if the application is hosted on a VM, chances are the VM can go down if the zone goes down. But these can be mitigated as you all are aware that all the service provided by Cloud providers eventually run on virtual machines.
Regional Services
These services are available in a particular region expanding to multiple availability zones within that region.
Most Cloud Providers have minimum of 3 availability zones in a region with some exception of 2 Az's in a region. With that being said the application hosted on the regional service can withstand atleast 2 zonal failures as the minimum AZ's are 3.
The best example would be Cloud Composer or GKE Cluster.
Most IT workloads make use of regional service as the chance of regional failure is very less and is economical, but still if your application supports live traffic or heavy workload with mission critical business, then opting for an service with multi regional or global would be preferred.
Multi Regional Services
These services are available or has the option to expand more than a region.
For consumers spread across multiple regions, multi regional services should be made use of. For an active-active or active-passive or for a DR activity multi regional would be a good option.
Best example would be Google Cloud Storage. While configuring GCS we have the option to create bucket as multi regional.
Global Services
There are only very few services which has a global scale availability and these services are mostly fully managed by the cloud provider with very less or no option to configure.
For business critical and higher reach for users across the global these services help a lot but is less cost effective.
Best example would be Google Cloud Datastore or Cloud Spanner.
Conclusion
Based on what business needs with how must would be the cost, the SLAs, SLOs defined by the cloud providers the services must be properly chosen as switching from one service to another is not an easy task.
Informative