Do a search on here and ls1tech under the pcm section. You'll find a lot of good info. A quick summary: while we do have maf's, the ve table is referenced during open loop. It's also referenced in closed loop at lower rpm and on throttle transients b/c the maf is slow to react to the sudden change in airflow. At 4000 rpm and up the maf is the source of airflow calculation. Below this, it's a mix of the maf and ve table. It's a good idea to tune the ve so that you don't have lean spots when you hit the throttle and it will also give you better throttle response.