A sponsorship agreement stating who pays the processing fee and the gift, whether any amount is repayable if she leaves, whether dependants are covered, and what happens if the application is refused; plus a dated copy of the official page the terms are based on.
The sponsorship agreement is a contract; treat it like one
Corporate sponsorship introduces obligations between employer and employee that the official page does not govern, so the documents that protect a Shannon applicant are the ones she negotiates. The agreement should identify which official track applies and quote its terms as of a stated date; allocate the nonrefundable processing fee and the gift; say whether the employer may recover any amount if she resigns within a period; state whether her spouse and children's fees and gifts are covered; and explain the consequence of a refusal or withdrawal. Hypothetical example: a software developer is asked to sign a five-year retention clause in exchange for sponsorship; she should have that reviewed by an employment lawyer, because the immigration benefit and the employment obligation are separate matters.
On the immigration side she assembles passports, civil records, prior U.S. immigration documents and anything the vetting instructions request. Keep the official page as read on the date, the signed agreement and every receipt together.