It should name a single employer, state the profession in the Appendix 2 wording, describe the professional duties, and explain the rotation as service delivery under that employer's contracts. Where the districts would each engage her separately, the letter cannot fix that; the contracts have to change first.
The letter records a structure, it does not create one
Hypothetical example: Imelda is a Callander social worker whose American offer would place her across three school districts. A support letter is often asked to smooth over an arrangement nobody has settled, and this is exactly the situation where that fails. The version worth presenting states the U.S.
entity's legal name as employer, Imelda's full name and Canadian citizenship, the profession as the Appendix 2 entry words it, the salary, the reporting line and the period requested. It then describes the professional work concretely: assessment, counselling, case formulation, safety planning, consultation with school staff, and the records she will maintain. The rotation across three districts belongs in that description as delivery under the employer's service contracts, supported by the contracts themselves.
If the districts pay her directly and direct her caseload, no wording rescues it, because the underlying structure is several employments and additional authorization would be required. Alongside the letter go the parchment, an official transcript and proof of citizenship, with certified translations for anything not in English. A single set of dates across every document is what makes the file readable in one pass.