Arm's-length sale
A sale on the open market between a willing buyer and a willing seller, with no force or pressure on either side. New Jersey assessors use SR-1A sale reports to judge whether a sale meets this test.
New Jersey example
Made-up example: one house sells to a stranger after a normal listing; another changes hands under financial duress or as part of a property trade. Assessors set the second kind aside for review. A sale marked non-usable for the state's ratio study can still be a fair comparable, like a new home that was improved after its last assessment.
Official source
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.