Register problems

Can i register for the server online or do i absolutely have to do it in-game?

Registering on the terraria server can only be done in-game in the Rift Dimension. Why, what is the problem you are encountering that this is a problem?

It said rep mode is enabled. I have no idea what that meant but i played on the server for a pretty long time and i tried again. After that it worked. What exactly is rep mode?