Confused.. Please explain

What is the difference between an RNY proximal and an RNY distal? What are the benefits of one as apposed to the other?

From my understanding the difference between proximal and distall is the amount of intestine bypassed. My doctor bases it on the starting BMI. Someone who has a BMI of 65+ may need more bypassed to achieve better weight loss where as someone who like myself has a 40ish BMI needs less bypassed. I hope I got this right. Mary