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 indoors and can be removed faily easily. The ourdoor life on Oracal 631 is 3 years. Oracal 651 has a gloss finish to it, unless noted matte. It is mainly used for outdoor use and has a 6 year outdoor life on Black/White and a 4 year outdoor life on all colors. The adhesive on it is more permanent, so it would not be very good as a temporary vinyl.