Primary and Secondary trade skillsFollow

I heard read that there are two types of trade skills. Primary and Secondary.

Primary, you can only have TWO of, like herbalism and alchemy, or mining and smithing.

BUT, Secondary skills, I heard that you can have and use as many as you can afford to work on, like fishing and first-aid.

I just want to know which skills are classified as Primary, and which skills are Secondary (the ones that you can have no matter what other skills you have)

Thanks,
kEL