Trade skills?Follow

Could someone please help me with some questions?

How does the primary/secondary profession work? Are certain jobs defined as primary/secondary jobs or is the difference in that you can't take more than two jobs up to max level?

If you do change job (I assume this works as people have said so) what happens with your old profession? Does it automatically go down and needs to level up in case you change your mind again and return to the first job?

Thanks!