SDL stands for Supplimentary Downlink. It may sound similar to SUL, but I think the motivation for SDL is a little bit different from SUL. The main motivation of SUL would be for improving the coverage by easing the UL power restriction in high frequency (refer to SUL page for further details), but the main motivation for SDL would not be for coverage expantion, it would be more for improving the utilization of spectrum resources which has been under-utilized

The frequency spectrum allocated for SDL by 3GPP are as follows :

The concept and use case of SDL  can be illustrated by Rahim Navael as shown below. Rahim kindly approved me to share the illustration in my note.

Image Source : Rahim Navaei's linked in post  

Reference

[1] A New Era for Enhanced Mobile Broadband - Mediatek/Whitepaper