Question : .fla images vs shapes

Hello Experts,

A friend gave me an .fla file of a rather simply animation to check out.  I noticed in the library for this .fla, there are two folders: images and shapes.  When I checked out their contents, I noticed that there seemed to be a 1:1 parody i.e. for each image it appears there is an identical shape.  

When I looked closer, I then noticed that the images were all odd numbered (Image 1, 3, 5, etc.) and the shapes were all even number (Shape 2, 4, 6, etc).

Questions: assuming I'm correct in this assumption, why would their be identical shapes and images?  What purpose does a shape serve in a time-line animation versus the purpose of an identical image?  Is this an unnecessary redundancy?  Why the odd/even split between images and shapes?

Answer : .fla images vs shapes

[19|20] means any character: 1, 9, 2, or 0 -- the vertical bar is redundant. The plus means one or more of the previous, so one or more of 1, 9, 2, or 0. The rest means exactly two numeric digits.

Try the following pattern instead:
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