Q:  Why do I need to distinguish between STRAIGHT or REVERSE COLLATED forms when using 3-part forms, but not with 2-part forms?

A:  Imagine that your forms are playing cards, with the Ace being the White Part (Part 1), the King being the Canary Part (Part 2) and the Queen being the Pink Part (Part 3). 

Here’s how a Straight Collated stack of cards (forms) would look vs. a Reverse Collated stack:

When using 2-part forms, one needs only remove the top sheet to effectively “reverse” the sequence.  As you can see above, the only difference between 2-Part Straight and 2-Part Reverse is whether the stack begins with an Ace (A) or a King (K).  Aside form that, there is no difference, as the stack just alternates back and forth between Ace and King.

Now consider 3-Part forms.  Using the same philosophy as the 2-Part, it would seem reasonable to think that you could just remove the Ace and the King form the top of the stack so that you’re starting with the Queen (Q).  However, a closer look reveals that the true sequence for Reverse Collated forms is Q-K-A.  This is NOT what you are left with when you remove the A and the K from the top of the Straight Collated stack.  Instead, you are left with Q-A-K, which is not the correct sequence for Reverse Collated forms.

For this reason, 2-part forms are only available in Reverse Collated sequence, because, as illustrated above, you can simply remove one sheet of paper from the top of the stack to switch it to Straight Collated.  3-Part forms, on the other hand, are available in both Reverse Collated and Straight Collated sequence, because to convert from one to the other would not require simply removing the top two sheets, but actually re-shuffling the entire stack of forms.