What is the purpose of a slip stitch?
Slip stitches allow you to join seams, at the waistband or collar for instance, hem garments, and enclose item without the presence of any visible stitches. This is useful if you’re working on a project that can’t be sewn by a sewing machine like a pillow seam for instance.
When would you use a flat felled seam?
Flat felled seams are used whenever a clothing item is going to see a lot of wear. They provide a very strong seam, and leave the inside of the garment nice and tidy.