It is a convention that was codified years ago, but provides little practical contribution. Basically it should convey that t the best of your knowledge, what you supply is not adulterated within the meaning of the code. Somehow the practice evolved from packaging to include food as well. If one were to actually read the code, it specifies that the company officer extending the guarantee reside in the United States (have a US address). Outside the country it has even less meaning. We continue to ask and be extended this guarantee as a futile but expected gesture. What really ought to be extended are policy statements, commitment statements and actual document copies that prove out your current food safety plans and certifications e.g, your HACCP plan summary with critical control points, your allergen control plan, your GMP audit summary and score, specifications, nutrition data sheets, organic certifications, COA, etc. In any case it's cheap and easy to comply with a Letter of Guarantee or Continuing Letter of Guarantee; if you're going to bother it should be on company letterhead with address, have a date and be signed by a company officer in the legal sense.