What constitutes an A international?
Basically an A international is a match involving the senior team. A Competitive 'A' International is a match in a FIFA competition (inc. Qualifiers) - & this is what would lock him into NZ. Tyler Boyd has played for the U20s (not 'A' internationals), and 5 friendly 'A' internationals (not competitive matches). Therefore he is still able to switch to the US.