To create a useful expression for a cell in a column, you will have to refer to other cells in different columns of the same table row. You can do this in two ways:
There is a special column whose name is "Index" and whose ID is "$0". The value of this is the same as the row number in the unsorted table (the grey numbers on the left of the grid in the Data Window).
The value of the variables so referenced will be a primitive
(boolean, byte, short, char, int, long, float, double) if the
column contains one of the corresponding types. Otherwise it will
be an Object of the type held by the column, for instance a String.
In practice this means: you can write the name of a column, and it will
evaluate to the numeric (or string) value that that column contains
in each row. You can then use this in normal algebraic expressions
such as "B_MAG - U_MAG
" as you'd expect.