I don't understand what happened. Why dont they know the job description?

Glitch in the world system? Or knowledge locked for now that becomes available when he reaches a certain level? I've seen isekai manga like that before so I wouldn't be surprised.

My guess is, since he simply wrote "rear guard" instead of a specific rear guard job, it gave him ALL the rear guard jobs at once. Likely the jumbled mess was all the job names written over each other. Just my guess though.

That's actually brilliant.