Subscriber/Publisher and dataReader/dataWriter Qos profile relations

[kgo4956]

fastDDS offers the possibility to define publisher/subscriber specific Qos like:
https://fast-dds.docs.eprosima.com/en/latest/fastdds/dds_layer/publisher/publisher/publisher.html#dds-layer-publisher-publisherqos

It offers also the possibility to define Qos profile for publisher/subscriber with API like: create_subscriber_with_profile

No profile is associated to subscribed/publisher in https://fast-dds.docs.eprosima.com/en/latest/fastdds/xml_configuration/xml_configuration.html
But it is supposed the profile is to create with DataReader and DataWriter xml profiles: right?
If yes, what is the relation between, for example, Qos profile of subscriber and Qos profile of associated DataReaders?
1) Without DataReader profile, subscriber profile applies
2) OR If datareader profile is defined, it overrides completly subscriber profile
3) OR it takes subscriber value when present if dataReader is missing (instead default value): "merge" operation
4) Other behavior?

In other words, when should a subscriber/publisher Qos profile be defined?