Difference between Mustahab and Sunnah Zaidah

I have come to know that both Sunnah Zaidah and Mustahab are not mandatory in islam and prophet(pbuh) encouraged us to do those. Then what is the (main) differences between those two terms. And if they are same why two terms are used and taught as two separate parts in islamic studies? I searched this online but didn’t even get any answers.