Ann: Exploration Update, page-9

An assay is used to accurately determine the precious metal content of a very small sample (~50g) in a lab, normally in batches of ~20 a day used for analysing drill samples, etc.

An extraction method is used to process large tonnages during mining (or trial mining) to extract the precious metals in a commercial plant.

BBX are using the extraction method as a proxy for an assaying method because we haven't developed an assay method yet.

Googling works...
https://investingnews.com/daily/res...ction-to-gold-assaying-whats-in-a-gold-grade/