Internal technical documentation the employee authored or maintains, training records showing she trains others, project files where her role is visible, and manager statements explaining why the knowledge is uncommon and how long a replacement would take to acquire it.
Show the knowledge in the company's own records
USCIS looks for evidence that the knowledge exists, that this employee holds it, and that it is not commonplace. The company's own documents are the strongest proof: version-controlled manuals with the employee's name in the history, internal training decks she delivered, tickets or project logs where she resolved problems others escalated to her, and product or process documentation marked proprietary. A letter from the Boischatel employer should explain the product or process in plain language, then describe the specific projects, then state the number of people in the whole group who hold comparable knowledge.
Add the U.S. entity's explanation of why it needs that knowledge now. Around this core, the standard L-1 file applies: the employment contract and a full year of payroll from the Boischatel entity, ownership documents linking the two companies, and proof both are operating.
If the work will happen at a customer's premises, include the service agreement showing the petitioner assigns and supervises it.