A: A split probability plot subdivides the data according to the colour attribute groups, and plots a normal probability plot for each group for the selected variables, on the same diagram (with the respective colour from the attribute dialog).  This allows quick comparison of distributions of subtypes in the data e.g. soil versus stream sediment, or drill hole 1 data versus drill hole 2 data.  If a group has a normally distributed sample population, then they will form a straight line.