Normally, worlds can prevent new character creation when they are temporarily congested at certain times of the day. This does not affect paid transfers, and is only temporary.

However, this was problematic, because it meant that people used paid transfers to get to "popular, always congested" worlds. To address this, Congested status was created.

Congested status prevents characters being created AND paid transfers and lasts indefinitely until a patch alters it. As cross-world features were added to balance world populations and the game grew, this status began being used all the time on lots of worlds.

So provided the status website does not say it's Congested, you can do a paid transfer to it. World Visit and Data Center Travel have their own, separate limits, which are reduced if the world is Congested but are still available. If too many people are visiting/traveling at that moment, it won't let you use it.