Prioritise the statement of work, service agreement, employer reporting structure and assignment description. Together they should identify the deliverable, location, duration, supervision and company-specific knowledge needed to perform the work.
Check operational consistency
A client confirmation may corroborate the arrangement, but it should not contradict the contract or actual working practice. Include employer evidence showing how managers assign work, review results and retain authority. Protect confidential information through a considered document strategy; do not remove the very terms needed to understand control or replace them with unsupported assurances.
Gather documents from both companies, since an arrangement described only by the petitioner tends to look better on paper than it is. From the petitioner: the client agreement and any statement of work, the employee's contract and reporting line, internal project governance, performance and appraisal records, and evidence of the methods or systems the engagement depends on. From the client side: whatever confirms the boundary between the two organizations, such as access arrangements, escalation paths and the client's own description of the engagement.
Check that the documents agree. Where a project plan names a client manager as the employee's supervisor while the petition describes internal supervision, that inconsistency is the point at issue, and it needs resolving in fact rather than in wording before anything is filed.