The recess is the part of the metal structure which goes up inside the lampshade 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, at the risk of seeing the bulb protruding from the lampshade otherwise.
If you need a smaller or larger fit, it is entirely possible. You just need to specify it in a comment when ordering so that we can send you a quote for the specific return.