Question about a rule..

[Sakurato]

k so I was reading the ban thread and I saw someone that got banned for "You're not allowed to play on someone else's account." <--- That.

Now I went to the "server Rules" thread...and I didn't find anything there about it, at all.

Is this an unwritten rule? Is it stated somewhere else? If not, can I get an exact definition of it? Did I just miss it in the rules thread, if I did I'm sorry