There are 4 BG layers in total, and since you only need two, it doesn't matter where you allocate the two others. It will always allocate 4. If you set the value to 3 for example, there will be 3 layers on the top screen and 1 on the bottom screen.
Oh wait, I'm totally wrong, sorry.
In your XSCREEN, you allocated all of the BG layers to the top screen, so you basically said BG wasn't allowed on the bottom screen.
The management number is the number you refer to when using sprite commands. A number from 0 to 511. This way when you use SPOFS with the right management number it won't pick some random other sprite.
The ID number is what your sprite looks like. It's from 0 to 4095, if I remember correctly.
Clues:
-The screen is 400x240 pixels wide.
-The command for placing a sprite is SPSET Management number, ID number
-The command for moving a sprite is SPOFS Management number, X, Y