The main difference between 651 and 631 is going to fall in the adhesive on the vinyl and the finish on the vinyl. Both are calendared films and can be used both indoors and outdoors. Oracal 631 has a matte finish on it - meaning it does not have the glossy sheen to it, it also has a lighter adhesive so it is best used
Expressions Vinyl - Oracal 631 12”x”12 Removable Indoor Vinyl Sheet
Oracal 651 Metallic 5ft. Roll Bundle
Adhesive Craft Vinyl Sheets - Oracal 651
Speedy Vinyl – Speedy Vinyl
12 x 24 Oracal 651 Permanent Adhesive Vinyl – Speedy Vinyl
WHATS THE DIFFERENCE: 631 VS 651
651 Oracal V.S 631 Oracal - Which One to Use and When – RQC Supply Ltd
Oracal 651 Vinyl Sheets & Rolls
5 ft Rolls of Oracal 651 Vinyl *Choose Color (12 wide) – Speedy Vinyl