Putting piping on the center skirt panel

I have a question about the instructions for putting the bias tape on the center skirt panel. In step #1 of that section, it says to baste the bias tape onto the sides of the center skirt panel. I assume with 1/2 inch SA because it doesn't mention otherwise. Then, in step 2, it says to machine- or hand-baste the piping in place using a 3/8 inch SA. What does this mean? Isn't it already in place from step one basting it on to the center skirt panel? It doesn't have you attach the side skirt panels yet, so I don't think it means that. What does baste it IN PLACE mean? Thanks!!!