I agree with GingerK; the letters are referring to the fabric. If you look on page 3 of the pattern, where the cutting instructions are, it assigns each fabric a letter. So instead of saying "make two 3.5" squares using the purple fabric" it says "make two 3.5" squares of fabric H".
Writing a pattern this way makes it easier to substitute fabrics, if you don't want to use the same colorway.