This is getting into market definition, which is kind of complicated and subjective, but one of the key factors is substitution.
So for example, is "Clorox bleach" its own market, separate from just "bleach"? Well, no. Clorox bleach is chemically identical to any other bleach. You could switch one for the other and not even be able to tell the difference.
On the other hand, is "broadband internet service in Pittsburgh" a different market than just "broadband internet service"? It kind of is. There may be a dozen different ISPs in varying cities across the country, but the prerequisite to using one that doesn't offer service in your city is to sell your house and move somewhere else. That doesn't make it a particularly viable substitute.
So then is "iOS app store" a different market than just "app store"? Well, what do you have to do to substitute one for the other? Is it reasonable to have to exchange your $600 phone, or buy a second $600 phone, in order to buy a $1 app from a different store?