The 30 or 27 Spline is referring to where the torque converter mates to the 700R4 transmission on the input shaft.
If you are ordering both the transmission and torque converter from Monster Transmission & Performance then you do not have to worry about this because we automatically match the correct spline for you. However if you are purchasing just the 700R4 transmission or converter you will need to determine how many Spline Count you have to match it up. To count the 700R4 transmission, you need actually count the teeth on the middle shaft (because there are 3 that stick out of the front of the transmission). Simply count the grooves / teeth on the middle shaft, it will either have 30 or 27 and that will determine what spline torque converter you will need to mate to your transmission.