Questions about macros in Roll20

I have two questions about macros. I've seen em referenced a few times here on the forums, and I'm not understanding em. The first one is the term "selected". I've seen a few macros set up on the forums like "/roll d20+@{selected|WIS_MOD}" Is this macro intended for the GM to be able to pull the wis mod off of the token he has selected? Or is the word selected a placeholder for the name of a character? My second question is in attack macros. I'll frequently see things like "/me recognizes an ally in trouble,looses a quick disruptive arrow [[1d20+13 vs AC]]" Is that intended to pull up the AC of the token you have targeted so you know if you hit? Or is this just designed to be a reminder that this attack is vs AC and not vs Touch AC or something? I'd be inclined to think it was just a reminder, but it's inside of the brackets for the roll so that makes me think it's referencing a number. Any help would be appreciated :)

In your first question, yes... it is intended for anyone to use to pull WIS_MOD off the selected token. In the second question, no. That is not going to pull AC off any token. It's just a reminder of what defense it targets.

Awesome, thanks for the info! I'm honestly blown away by this community, guys. I've asked like 4 questions and been given an answer to all of em within the first hour of em being up. You guys are great for sharing your knowledge so quickly and freely. It's incredibly refreshing to not just be told to RTFM, haha.