How an engagement opens
Counsel can engage me directly and bill it through, or the party can engage me on counsel's advice. Either works. The engagement determines who my client is and who is entitled to rely on the report, so it is settled in writing before I begin rather than negotiated after the report exists.
Before I accept, I run a conflict check against prior work. USPAP requires me to disclose any service I have performed on the same property within the prior three years, so send the property address and the names of the parties with the first call and I will tell you quickly whether I am clear to take the assignment.
Three things open the file: the intended use, the intended users, and the effective date. Those three define the assignment, they shape the scope of work and the data I analyze, and none of them can be retrofitted once the report exists.
A past effective date is routine work, not an obstacle. Date of separation, date of death, petition date, January 1 of a tax year. The market data from that date still exists in exactly the form it always did, and the report states the retrospective basis plainly.