Consumers and Associations

Can you tell me what is the best practise for identifying service consumers? In our layered SOA architecture we have services being consumed by both applications and other services.

Ideally I would like to go down to the service operation level but I can only add application consumers and am unable to create a ‘consumed by’ association for an operation. This is fine if the consumer is an app but if the consumer is another service operation I am unable to make the association at the operation level.
Is the standard practise only to identify consumers and associations at the services level ?