The petition needs the service contract or statement of work, an itinerary of services with dates and locations, and evidence that the petitioner directs and supervises the worker. Terms showing the employer sets the method, owns the deliverable and prices the work as a service rather than as hours are what distinguish a placement from labour for hire.
The contract with the customer is the document that decides it
Where the assignment happens on an unaffiliated company's premises, the file has to answer a question the ordinary petition does not raise: who is actually in charge of this person. The service agreement usually answers it, for better or worse. Helpful terms define a scope of work and a deliverable, reserve the method to the employer, place the employer's own supervisor over its personnel, and price the engagement against milestones.
Unhelpful terms describe personnel by headcount and rate, allow the customer to reassign the worker, or make the customer responsible for day-to-day direction. Alongside the contract, an itinerary listing the services, dates and locations is expected where work will be performed at more than one place. Internal evidence matters too: the reporting line to the petitioner's own supervisor, the employer's project plan, and the method documents the worker will apply on site.