In the geosteering window StarSteer switches TVT scale from TVD TVT stands for true vertical thickness, or so called "stratigraphic thickness", sometimes in the literature it can be called KBTVD. It is the bases for the strat-based geosteering approach that is used in StarSteer.
For a vertical well (typewell, pilot hole) TVT is the same as TVD with the correction to the shift you may apply in typewell referencing mode. Shift is specific for every typewell-lateral pair and is defined in the typewell referencing mode. Every typewell will have its own shift relative to the specific lateral. Reference marker is one value and is defined for the lateral.
For a horizontal well it is transformed from TVD to TVT by a special algorithm so it is possible to compare (correlate) it with typewell. That's why in TVD scale (no mode) and TVT scale (geosteering mode) your lateral curve looks different in interpretation window.
StarSteer is very flexible and allows you to geosteer even if you haven't defined shift and reference marker. Your geosteering interpretation will still be correct because you embedded the desired typewell shift to the shift of the first segment of your interpretation.
However it is not recommended to do it this way because it makes it very difficult to perform the multi-typewell geosteering when you have several typewells with different shifts.
The recommended way - is the way with shifts and RM defined.
See below the new training video on StarSteer's typewell referencing for more information: