Frames.
The die cuts outside and inside and you end up with a piece that looks like a picture frame with a line of stitching on the frame.
In the picture
here (your link) the blue plaid frame around the opening on the card is made with a stitched rectangle frame die.
Stackables.
The die cuts the outside of a rectangle with a line of stitching near the edges.
In the picture
here the blue rectangle is cut from a stitched rectangle stackable.
4 Bar.
This is a card size, like A2 is a card size. 4 Bar cards are 3-1/2 by 4-7/8 inches, fit an envelope 3-5/8 by 5-1/8, and are the smallest mailable size of card. The dimensions have different proportions than the A2 so the die sets made for layering have to be different.