It would depend on the reception and the dress code. Semiformal (evening), jacket and tie, jacket does not necessarily have to be part of a full, matching suit but a standard wool suit is a step up. Semiformal (day), I'd say you can ditch the jacket as long as you have a nice dress shirt and dressy pants.<quoted text>
You're hung up on this suit business. For me, a jacket and tie qualifies as "semi formal." I don't know why you think that wouldn't be appropriate for a wedding reception.
Black tie, pants and jacket must be part of a matching suit, preferably tailored with decorative/contrasting/silk lapels.
White tie, tuxedo required.
And once again, that's why it sounds like the letter writer is actually confused, or the person who gave him the instructions on dress code is confused. Someone said semiformal but the instructions given are seem to indicate at least black tie.