These are in SA\X360. They seem to control which textures are loaded for various stages. I suspect they define memory allocation for textures.

Demo: Used for menus?
Mercenaries: Used for the entire mercs mode (and other extra content modes?)
Game: Seems to always be loaded before loading any level.
S*: Loaded for specific stages. (counts for events as well).
E*: Loaded for specific events.

Tests with blanking out parts of SPKG data (used Mercenaries.spkg for testing)
Fixeddatasizechunk: Almost everything black. Certain lights rendered.
Part 1: Crash
Part 2: Crash
Part 3: Everything became blank except for effects
Part 4: Crash
Part 5: Most stuff rendered with black. Some elements render, but are just bloomy neon colourful shapes.
Part 6: No change (duh!)
Part 7: Same as fixeddatachunk.
Shaders: Same as fixeddatachunk.

-Re arranging entries in fixeddatasizechunk causes stuff to not render properly anymore.

Comparison between mercenaries.spkg from release version, and same spkg from version after L4D chars were added:
These are the various entries added:
packageCount + 67
part1Count + 776
part2Count + 443
part3Count + 0
part4Count + 0
part5Count + 29
part6Count + 0
part7Count + 70
fixed-size Data Chunk got new 15 values which aren't 0. It's interesting to see that the order of zeroes and non-zeroes is the same, but the actual values are different. Maybe the shaders/packages were oredered differently when they re-created mercenaries spkg for this patch which made these values different.