The RelatedToAllParents field is one of the set of RelatedTo fields which can be used to search the "RelatedTo" (Destination) end of a relationship.
When querying relationships in a sub-select, RelatedToEntityType can be used to filter the RelatedTo end of the relationship to only items which are of a certain entity type e.g. Requirement, Script etc.
The RelatedToEntityType field works in the same manner as the EntityType field, refer to that topic for a list of valid values.
All requirements related to open bugs (EntityType of Incident)