It should name the profession, describe the duties, identify the credential relied on and the employer, state the expected period, and explain the nature of the professional work at each anticipated location rather than simply listing the states covered.
Make locations describe work, not reach
Where a project schedule or site list already exists internally, attach it so the anticipated locations have a source. Then read the letter against any contract, statement of work or client agreement covering the same role, since a letter describing advisory work and a contract describing something else will be read together. Reconcile the two before either is relied on.
Set the letter beside the record that could contradict it. An offer letter states what the employer intends; a statement of work, purchase order or client services agreement states what was actually bought, and where those two describe different activities the second usually carries more weight because a third party signed it. Ask which document would surface if the assignment were audited internally, then make sure that one agrees with the letter.
Hypothetical example: a computer systems analyst is assigned to configure a customs-brokerage and trade-compliance platform at several client sites, while the client agreement describes hourly staff augmentation. Reconcile the difference before anything is filed, since the analyst's duties and the question of who directs them are exactly what the discrepancy puts in doubt.