VW diesels are a little unusual -- the pistons poke up above the surface of the engine block:
This is due to the extremely high compression ratio of 22.5:1 (your gas car is probably between 8:1 and 9.5:1) -- it needs to squeeze all the air into as small a volume as possible.
Due to this piston projection, you need to keep the pistons from crashing into the cylinder head. As the amount of piston protrusion varies based on your main bearings and wrist pin bushings (you recall I just replaced those), VW supplies three different thicknesses of head gaskets. Why not just use the thickest for maximum clearance? Well you can, but the lowered compression ratio will make your car hard to start in winter. So you actually want as thin as possible for cold start performance without being too thin and things hitting each other. I measured my piston stick-up and determined that I needed the thinnest gasket available. This made me a little nervous, but after I bolted the head on
and installed the timing belt
I was able to turn the engine over just fine without the pistons bumping into anything. Whew!