The geometric shape denoted which bomb division the group was part of, i.e. triangles were in the First Bomb Division, squares were the Third Bomb Division.
The letter within the geometric shape denoted which group the plane belonged to, so the letter "A" for example could be either the 91st or 94th Bomb Groups – if the "A" is inside a triangle it's the 91st Group (part of First Bomb Division, if it's inside a square it's the 94th Bomb Group (part of Third Bomb Division).
The individual planes within an element should all be from the same Bomb Group, so all should be (for example) Square D – the 100th Bomb Group.
The difference in markings within an element would be the individual aircraft designation letter (which aren't on the I-94 sheet you're talking about). Each plane had a 3-letter code, the first 2 letters denoted the squadron (each Bomb Group being composed of 4 squadrons), the third letter was to designate the individual plane. In most groups the individual plane letter was also repeated on the tail below the group insignia.
Also note that the group insignia appeared on both sides of the tail as well as on the top right wing.
Hope this helps, shoot me a PM if you have any more questions – I've got the decal sheet you mention, and LOTS of reference material for the 8th AF.
reports from OrcCon 2008.