What 10 placement matches actually decide
At a season reset Overwatch has you replay ten placement matches per role before it re-settles your rank. This is not a wipe back to Bronze: the game carries your hidden matchmaking rating over from last season and uses the placements to re-confirm where you belong, so a strong run nudges you above your old rank while a rough one settles you just below it. Because each role holds its own separate hidden rating, you place each role independently — your Tank result says nothing about where your Support lands. A placement boost plays those ten games while your carried-over rating is at its highest leverage.