What constitutes proof of entitlement?

Perhaps if we examine a definition of proof of entitlement, then we can further qualify the statement below:
“Proof of entitlement is the necessary evidence a software vendor requires of a client to demonstrate their rights to install and use software.”
Suitably ambiguous for you?! Perhaps so, however if you think of the number of software vendors you deal with, each one will ask for different items depending on the software title in question. Some typical examples of evidence might include:
Software Licenses: The most obvious example, however most software vendors will insist on supporting evidence to demonstrate a sale took place to back up a license document – equally, many purchases of software won’t necessarily come with a gilt-embossed piece of parchment offering unquestioning rights to use.
Contracts: Vital pieces of evidence, not least in drilling down into the finer points of deployment and use. Standard licensing terms and conditions could be superseded by such documents, and so are worthy of secure storage and retrieval within a document management system.
Invoices/Receipts/Purchase Orders: Many software vendors will request some evidence of a financial transaction having taken place. Arguments have been made as to the relative value of Purchase Orders vs. Receipts etc.
Certificates of Authenticity (CoA): In the Microsoft world, part of the overall evidence required to demonstrate certain installs of software. In isolation, a CoA is not a license, merely a sticker to verify that an installation of Microsoft software is the genuine article.  However, it is required in support of the license to cover the relevant software installed.  DO NOT peel these off and attempt to keep them in a safe place!
Packaging/Documentation: Depending on the retail channel you go down, you may have boxes, and supporting documentation that you are required to retain AND original media also.
Emails/License Keys: More for auditory purposes, such emails can demonstrate that a license key was emailed from an authorized source.
Will scans do? For the purposes of expediting an audit, possibly so – but have the original documentation to fall back on – software vendors are not duty bound to accept scans; after all you could have been getting creative in a graphics package to generate the scan.
This list is not exhaustive, so check with your supplier/software vendor as to what evidence should be required; Proof of entitlement could even vary within one software vendor’s portfolio if their product set is wide enough.