Hola David y Paris

OK, I can see it in context now, thanks. 

The whole sentence is:

"Los que hayan visitado la ciudad de Barcelona habrán podido admirar la herencia magnífica de Gaudí."

This is a relative clause using "los que" (those who.., the ones who...). What comes after can use both indicative or subjunctive, depending on what it conveys each time. 

In this case, the use of the subjunctive indicates that this is "unknown information" by the speaker, i.e. they're not referring to a specific group of people that they know for sure have visited Barcelona, but people who may have or may have not visited Barcelona, as in saying "those who may have visited Barcelona...(whoever those are).

If the indicative had been used instead: "Los que han visitado..." we'd have to assume that the narrator has in mind a specific group of people who have indeed visited Barcelona.

Here is a Kwiziq lesson that talks about this topic and explains it. Have a look:

Spanish relative clauses with the indicative and the subjunctive (subordinate relative clauses)

I hope it clarifies your doubts.

Saludos cordiales

Inma

Hola Paris,

I would need to see the rest of the sentence to know why. It is a subordinate clause using a relative, but would have to see the whole sentence. I can't see any using this in this lesson that is attached to your question. Can you tell me where you saw it so I can give you a precise answer please?

Inma