Per Awingu domain
These DNS servers are used to resolve servers matching the "FQDN for UPN" defined for the Awingu domain. Multiple servers can be entered comma separated. E.g. if FQDN for UPN is domain.internal, then fileserver.domain.internal will be resolved with the mentioned DNS server
Global DNS servers
These DNS servers are used to resolve all other requests.
It's also worth noting that the global DNS servers are used for reverse lookups of IP addresses. Make sure that the global DNS server is pointing to a DNS server that is capable of resolving all reverse DNS lookups for all tenants/domains using Single Sign-on (SSO).