Charlie,
You are correct except that mylar wrappers are available pre-cut for 10 and 12 gauge shells. They are available in thicknesses of .005" and .010". Their purpose is to of course increase the thickness of compressable material, such as the wad petal, between the hard shot and the softer barrel. Mylar also reduces friction which should in turn decrease pressure.
Frank,
The x10X is a gas seal that in some loads is combined with a mylar wrap to replace what would in other loads be the wad.
Load #90 calls for (1) [thanks to Pete's correction below] x10x gas seal, (2) 1/4" filler (16 gauge wads or portions thereof) and MR (mylar wrap).
Mark
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