I think I get it. You can use it to talk to people, or write messages to them, so long as they have one of these too.

[He pauses, and frowns thoughtfully.] How far do you think these can communicate? I'm thinking of getting some for my friends.

[Aqua's Wayfinders are irreplaceable, but if they could call each other any time, anywhere, they really wouldn't ever be far from each other. ...That could be both a curse and a blessing, given how much he's tried to avoid his friends lately. But being able to check up on them at the push of a button is a tempting idea.]