The scope letter goes to the general contractor with the price, or just before it. Many subs send the scope early and the number on bid day.
A fire protection sub might include design, permits and wet system piping. It might exclude the fire pump, the underground main and painting of pipe.
Name the drawings by date. Name each addendum you received. List the spec sections you cover. Put exclusions and qualifications in their own lists.
Keep the wording the same from job to job where you can. Estimators at the GC then learn how to read your letter.