Number 9 envelopes are commonly referred to as a business reply or return envelope? Why? A #9 envelope fits easily into a #10 envelope making it perfect for when you intend on someone sending something back to you. They are called regular because they do not have a window. If you choose to print on these yourself with an address for return they are inkjet and laser printer compatible.
Tinted or security tinted envelopes have printing on the inside of the envelope to help eliminate the ability of someone to see what you are sending without opening the mail piece. Are you mailing sensitive information? If so, you will want to consider adding the security tinted option to your #9 envelope order.