The recess is the part of the metal structure that goes up inside the shade to support the bulb.
On classic structures, this recess measures 4cm in height, so be careful to take it into account in your measurements, especially for a small lampshade, otherwise the bulb may protrude from the shade.
If you need a smaller or bigger recess, it is possible. Just specify it in the comments of your order so we can send you a quote for the specific recess.