In page 40, the class AddressViewController define:
public var address: Address? //Model
For me, this is not a strong reference to the model, so it should be “use” or “depends” or “delegates to”.
At being optional it could be injected when needed. even further, it could be a protocol so you are not going to programming to an instance but a protocol.
Please, could someone explain it? Thanks.
@andreskwan Thanks very much for your question!
In Computer Science/Object Oriented Programming terminology, there are two types of relationships that are defined:
It is for this reason the address variable would be referred to as a “has a” relationship. Moreover, these terms are quite old, and very generic, so they predate Optionals. I believe the term “delegates to” is an incorrect reference to this property, since “delegate” has a very specific meaning in iOS.
The class HAS an address property. Whether it actually USES it (because it is nil) is another matter. The fact that a property can be optional should not negate the fact that it is a property belonging to a particular class. An example would be a bank account that belongs to an individual. This bank account is allowed to carry an amount of $0.00. The fact that the account is allowed to have no money does not negate the fact that it is still a bank account, which belongs to a particular person. 
I hope this helps!
All the best!
@jrg.developer Can you please help with this when you get a chance? Thank you - much appreciated! :]
Thanks for your question. 
All in all, the reply from @syedfa is pretty spot on! Specifically,
It doesn’t matter whether the property is optional. If a class “has a” strong property (see note later about “uses” for weak), you nearly always show this as a solid, “has a” line.
By the way, this IS a strong reference, but it’s an optional type. That is, AddressViewController owns an Address, but it may not yet be set and be nil instead.
There is a “uses” relationship too — shown as a dashed line with solid pointer — but this often is used to indicate either (1) the relationship is to a weak property or (2) there isn’t a property relationship but rather it’s a type that’s “used,” such as a method input parameter but not held onto.
Since there is ambiguity, “uses” relationships are typically annotated for what they mean, via text next to the line.
Make sense?
Yes, it does. Thank you.