no# 15 is a full back in rugby union
winger
jarrod hayne
loosehead prop
Its someone who can play centre across all the positions in a rugby teams back position
Usually number 8
the ugly raped woman
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.