Screen cx offset of hit alien. Aliens are 2 bytes wide so this goes 0, 2, 4, etc. So hit the the leftmost alien this will be 0, the one next to it will be 2, etc. Divide this number by 2 to get the number of the alien which has been hit, 0 to 10, counted from the left side.

This holds the score value of a hit alien divided by 10. So a green alien, worth 20, would put 2 in here.

Looks like a bug here. _EXPLOSION_CX_CY should be 2 bytes of storage for the bullet cx,cy location at the point of making the alien explode, but _COLL_YAXIS_TEMP_STORE shares that second byte. Clearly the temp store thing doesn't clash with the bullet location information otherwise the explosion would appear in the wrong place.

Y axis collision code stores A register here

Address of top row of explosion graphic to be drawn is placed here. It'll be the UDG_TOP to draw it and UDG_CLEAR to clear it

Address of bottom row of explosion graphic to be drawn is placed here It'll be the UDG_TOP to draw it and UDG_CLEAR to clear it

UDGs, interleaved. It's a 16x11 image, so 0th row bytes 0 and 1, then 1st row bytes 0 and 1, etc. The bottom 5 scans are empty.