What's a Warm Up Set?
Strong lets you label Warm Up sets separately from your working sets.
Warm Up sets (as opposed to working sets) are usually used to warm up muscles prior to working sets, which require more exertion and effort. Warm Up sets shouldn't significantly exhaust the body prior to working sets.
In general, Strong ignores Warm Up sets for calculations, such as total workout volume, and when determining your best sets and personal records.