Beyond the business file, keep the marriage certificate, the spouse's Canadian employment contract or resignation notice showing the expected end date, housing arrangements on both sides, and a travel record for the household. These explain why the family is separated for a period and support the later dependent application when it is made.
Document the marriage, not just the household
Dependent eligibility rests on a legally married spouse and unmarried children under 21; living together or sharing finances does not substitute for the legal relationship. Keep the certificate, any name-change records, and translations if needed. If the spouse will later seek work authorization incident to status, plan for proof such as an E-2S annotation or the I-94 record, and confirm what the employer or agency requires at that time.
Ask of each document what a reader would conclude if it were missing. A marriage certificate is not optional, because the dependent relationship is legal rather than factual and nothing else establishes it. A resignation notice bearing an end date is different: it is useful because it explains a separation that would otherwise look unexplained, but its absence is not fatal, and a letter confirming the employment and its expected duration may serve the same purpose.
Sorting the file on that basis puts effort where an absence would actually cost something. Bear in mind that the documents supporting a spouse's later work authorization are assembled at that time and against the requirements then in force, rather than gathered years in advance.