The official program website and the executive action that established it, together with any procedural guidance published by the responsible agencies, and the USCIS fee schedule for any related filing fees. Those are the sources that state the terms; anything else is commentary.
Go to the primary materials and date what you read
When a program is new, the most common error is relying on a description written when the terms were different. Read the official page and the executive action yourself, note the date on which you read them, and re-check before acting. Where a point is unclear on the official materials, treat it as unclear rather than filling the gap with an assumption from another category.
Keep a copy of what the page said on the day you relied on it, which is simply prudent record-keeping for any decision involving a substantial payment. If a professional adviser is engaged, ask them which official source supports each statement they make, and be cautious of anyone who cannot answer that question directly. Hypothetical example: an archivist prepares a file after a corporation offers to support her application.
She should first obtain current official instructions stating who may pay, what proof is required, and whether any sponsorship record is needed. Preserve passports, civil records, proof of payment, payment-source documents, and the evidence for the underlying immigrant criteria. A first review should date each official source and identify gaps.
Problems arise when an applicant retains a receipt but cannot show why the payment was made, who made it, or how the rest of the eligibility record is satisfied.