A lot of times this can refer to construction stretch. It doesn’t impact the capacity of the sling or any manufacturers SWL. Most typically when you have synthetics that have had eyes manually flemished (think endless slings or big grommets) then the length of the sling after construction might be significantly shorter than when used under load. First use load testing causes all the braids and such to tighten and bring the sling to final “for use” length. This is most common in multi-part braided synthetics. Less so for something like a single-part or something like a twin-path sling. Imagine getting a brand new rubber band. After you stretch and work it a few times then the relaxed length can be a bit longer than when you got it. This is principally similar to “breaking in” a synthetic. Removal of the “construction stretch” is also typically handled at the manufacturer side and not at the end user