What is the difference between TVDSS and TVDRT?

TVDSS -- True Vertical Depth Sub Sea. All depth with reference TVDSS are relative to the sea level, where sea level = 0.0.

TVDRT -- True Vertical Depth Rotary Table. All depths referenced to TVDRT are relative to the rotary table on the rig (same as KB, Kelly Bushing).

This image compares 2 wells, one offshore and one onshore well with given KB values for both wells.