J-trim is an optional strip of metal, shaped like the letter J, that can be added to the bottom edge of a regular-style roof, or to the bottom edge of a partially-closed side.
There are several benefits of adding J-trim to your carport:
It protects people from getting cut by the sharp edge of the metal sheeting
It makes your carport slightly stronger, by helping lock the sheeting in place
It add a nice appearance to the edge
I strongly recommend J-trim if you have partially-enclosed sides in order to protect children from getting injured by the sheeting.