I measured a traverse with Access and measured station and target heights to the bottom notch. But I used a custom prism constant.
Now, when I drag'n drop the Job file into TBC 5.20.2 it complains
That is nonsense, and from my point of view a bug. If I make sure I have prisms with the correct distance above the tribrach and select that method in Access then TBC is not to change my measurement method, it's not even asking. That is messing up my elevations.
That message is actually confusing and should be rephrased. I also showed that message to my native English speaking colleague and he also would think the message means an i.e. 1.40 m bottom notch is used as 1.40 m true vertical height. Instead TBC does actually convert the value according the formula from the S series manual.