When looking at material data sheets you may see 'Elongation at Break %' listed as a spec.This number is expressed as a percentage of the original length of the product at the point it breaks.
Elongation - The point of failure of a given material or the distance an adhesive will stretch before it breaks.
You can see in the video below how elongation is measured. In this video they are not testing elongation at break but elongation at a given weight. (Begin around 0:53 on the video)
Elongation is important to adhesives for several reasons. Many masking applications require adhesives with good elongation. If the adhesive can stretch while being applied to a surface it helps the applicator to be able to mask small corners. These tough to reach spots are important to mask otherwise they can be exposed during the paint process. Ford uses Strouse tapes on their clay models when designing cars. They often stretch this tape to meet the contours of the body and to create the design they are looking for. The flexibility of the adhesive gives them the freedom to communicate their ideas instantly.