Grades have to be defined in Survey->Data precision
Here you can define standard deviation for dataset so you can bind it to session.
How it work? If you need to span errors in loops not on all shots but more in some shots and less in others (for example due to different survey's tools or technique) define 2 precision sets with small sd range and big sd range, bind it to shots using sessions and see the result.
cSurvey relay, for this, all on Therion so you can read the Therion manual about sd and grade command.
See attachment for an example.
