National Instrument 43-101 is the Canadian standard governing how issuers disclose scientific and technical information about mineral projects. Its core requirement is that material technical disclosure be prepared by, or under the supervision of, a qualified person, and supported by a filed technical report. It exists so that a grade or a resource in a press release is not just a company's word.
The place it protects investors most is the phrase 'historic resource.' When a company describes a historic or non-43-101-compliant resource, it is explicitly telling you the number does not meet the current standard — it may predate modern rules, rely on lost data, or use methods that would not pass today. Issuers are required to caveat such figures and state that they are not treating them as current. Many investors read past exactly that sentence.
The rule of thumb: a compliant resource is a starting point for analysis; a historic or non-compliant one is marketing until a qualified person re-establishes it. Treat the two completely differently, no matter how large the historic number sounds.