Quote: pie.warrior "both 5/8ths now look at the opposition and try to place themselves in the attacking line up to exploit the weaker defenders to try and make a line break or pull a 2nd defender in to cover and thus creating an overlap. The coaches analyse the opponents and with the playmakers of their own team (1,6,7,9,13) formulate a plan A,B,C. even in the same game you will find that the 5/8ths often change their style and positioning of attack to exploit weaknesses.'"
Only the stand off is (to use the aussie parlance) a 5/8, so named as he will stand five eighths of the way back between the scrummage line and the last man (the full back). Scrum half is a half back, standing half (or 4/8ths) of the way back. Similarly the wingers and centres stand 3/4 of the way back and are called threequarters and the full back stands the furthest back so is all the way (full) back. That is how the positional names came about during the formation of the game in the 1800s.
As others have said different teams will apply different formations and give different roles to their SO and SH dependent on what the skill set and strengths of those individual players are. There no longer is a specific fixed role for either position as their roles are largely interchangeable.