Buy means you pay for it yourself and order it there and then.
Request means you put it up for Public Brokering, and someone might order it for you.
The advantage of this is you pay only a small amount upfront up front and then the rest when you buy it off the broker.
Also, you have three months to pay, so it makes it a lot easier if you're short on cash
The problem is, in order to make brokers want to take your request, you should put it up with a positive markup, meaning you pay more in the long run.
Depends on the world you are in. If the world you are in have been running for a while, you should take a look at the A-Z Broker Market in this forum to see what discounts the official brokers are offering. Also, buying new planes is not necessarily the best option at times.
