Number 22 in Rugby is a player on the reserves. Therefore it could be occupied by a player of any specialised position. However, the player in number 22 is usually an outside back utility(playing centre, wing and fullback)
no# 15 is a full back in rugby union
Usually number 8
the ugly raped woman
Its someone who can play centre across all the positions in a rugby teams back position
Formally Phil Vickery was a prop
Wing for RC Toulonnais
He plays in the openside position for the New Zealand, Crusaders and Canterbury rugby teams
He plays as a reserve and comes off the bench.
There is no compulsion to "choose" a position. As you play certain factors will come to light suggesting what your best position might be - size, speed, skills etc.