Hi, Vladas,
Let's look at relationship Customer-HomeAddress.
Customer must know his/her address, but Address does not need to know anything about it's customer.
Here is the piece of DD for this relationship:
It was taken from
SCBCD Study Guide web-site.
As you can see, Address definition does not have cmr-field element. This means, Address have no idea about customer (the relation is unidirectional)
This and much-much more you can find
there Cheers!!!