What is the difference between a # 6 3/4 Regular envelope and a # 6 3/4 Remittance envelope?
Both of these envelopes have the same final size of 3 5/8" x 6 1/2", or 3.625" x 6.5".
The difference is the size of their flaps and their utility
#6 3/4 Regular: 3 5/8" x 6 1/2" - Commercial envelope with typically a 1.25" diagonal flap.

#6 3/4 Remittance: 7 1/4" x 6 1/2" when flat, 3 5/8" x 6 1/2", or 3.625" x 6.5" when closed - A Donation envelope with expanded areas to print on the oversized flap which is 3.375" tall

Attached are the templates for a # 6 3/4 Regular and a # 6 3/4 Remittance Envelope