You can use the entity selector on the Relation target entity!
As an example, suppose you have multiple namespaces in Azure and you have the same name for a service bus queue in these namespaces.
You need to tag these queue's using below entity selector.
It looks like a complex query but if you read,
select this type, with this relation (but select only these target entities)
@henridb : thanks for sharing!
fromRelationships and toRelationships are great, that's for sure.
For those that want to know where to get all possibilities, just check with https://www.dynatrace.com/support/help/dynatrace-api/environment-api/entity-v2/get-entity-type