When assigning tasks, you can filter resources based on their certifications. This allows you to accommodate different clients and sites (locations) that may have different certification requirements.
When creating a Task against the same Client and Location, click Task is Assigned To to filter the user list with one of two additional columns: 'Certified (Task Client)' and 'Certified (Task Location)'.
These fields allow you to restrict your users list based on which users have the certificates required by the Client and Location. This list will also filter Sub-Contractor suppliers to only show those that meet the requirements.
This filter is also available in the Calendar when scheduling Resources.