A queue in most MMOs is a mechanism that controls population in a server, keeping it at a hard cap. Working with examples: let's take the 7.500 cap as it was stated now we have on our worlds. So... people keep logging in normally, as long as there's less than 7500 people inside. if person #7501 tries to login, they'll be put as first place in a queue, the next person will be #2 on the queue, and so on. So, if anyone logs out of the world, the queue system puts Mr. 7501 inside, and everybody in the queue moves up a slot. That way, as people log out, people are let in, server population never goes past the hard cap, people eventually gets in, and everyone is happy. The player, who'll eventually get in just having to wait a while; the game, who won't crash for having too many people connected; and the company, who won't have to put up with people complaining about "server full" errors
Now, What I'm saying here is that FF14:ARR does NOT have such a system in place. What it does have is a system that "funnels" people trying to log into a server at the same time into a queue, pushing them to the world server one by one. Nothing else. So, when you get past this "queue", one of two things happens: the server is not full, and you get in, or the server is full and you get a 1017 on your face. So, when they say they are "limiting logins", I take that means only that they, by default, don't have any kind of cap on how many concurrent users are allowed in the worlds, and when they are "activating login restrictions", it merely means they flipped on the switch that puts the 7500 max pop, blowing additional connections off with a 1017.