<<Disclaimer:  Verify this information before applying it to your situation.>>

>Help - I am new at this - A recipe from Betty Hagman Cook Book says 2 Cups
>of GF flour which means 2 parts white rice flour - 2/3 part potato starch
>and 1/3 part tapioca flour - What does this mean Parts?

In place of the word "parts" substitute any familiar unit of measure:

2 cups white rice flour - 2/3 cup potato starch and 1/3 cup tapioca flour.

Although Bette's proportions are based on volume measure, the flours are
close enough in density that you can substitute units of weight with good
results, e.g. let "part" be 100 grams:

200 grams white rice flour - 67 grams (2/3 X 100 g) potato starch and 33
grams (1/3 X 100 g) tapioca flour.

Hope this helps :>)

Joe S. Warren
Titusville, FL