Redacted internal documentation, training and certification records, project summaries naming the employee's role, and a letter explaining the process in enough detail to be understood. The employer decides what to withhold, but must leave enough to prove the claim.
What to ask the employer for
Ask the employer for a document inventory before anyone drafts a letter: which manuals, specifications or software configurations embody the process; which training records show the employee learning it; which project files show her applying it; and which of those can be shared in redacted form. Hypothetical example: a Lévis actuarial-software team at an insurance group built an internal pricing-model validation framework, and its lead analyst is to deploy it at the U.S. subsidiary.
The framework's code is confidential, but its architecture document, the internal training curriculum, the analyst's completion certificates and two project post-mortems naming her as method owner can all be shared with sensitive figures removed. Pair those with the corporate ownership records and her payroll history for the qualifying year. A letter that describes the framework in general terms without any supporting artefact reads as assertion; the artefacts are what carry the weight.