A dated copy of the official terms relied on, complete payment records identifying the payer and the account, and any written arrangement with an employer or intermediary setting out who bears each amount. Nothing that promises a result.
Keep the terms as they stood on the day you decided
Hypothetical example: Josiane owns a road-construction company near East Ferris and is weighing the program on figures an adviser quoted eight months ago. Program terms are published rather than legislated, and published terms move. The first document in her file should therefore be a dated copy or print of the official page as it read when she made her decision, so that a later discrepancy is a documented change rather than a dispute about memory.
Payment records come next, and they should show the individual applicant as the payer where the application is individual, since money routed through her company creates questions in two systems at once. If an employer or intermediary is involved, the written arrangement should state who pays the processing fee, who bears the gift, and what happens if she leaves. She should also keep her own eligibility evidence for the immigrant category the application will use, because that is where the decision is actually made.