Dumb Question – can't find the answer

Hi Guys!
Hate to trouble you with questions if the answers are in the (very easy to follow and well executed :-)) documentation.

I’m importing a lot of listings and notice the categories are by id, but in the field it says ,125, and I realize that the default category must appear in the categories.

Question, Must it appear in this format ,125, with the commas on each side? meaning do you need the initial comma?

Can we use the category slug? like instead of 125 use category-slug-1,category-2-slug?

I’m cleanining my listings in CSV and adding the categories so they import into created ones.
Just checking.

Just got hired to build another directory and will get them to buy their subscription. Sold one for you!

Mark