When Hero Pools come back, they will be active in your match if the average SR amongst all the players is greater or equal to 3500 SR. So if you’re at 3495 SR, you could be in a match with hero pools if other players in the match have higher SRs and raise the average above 3500. If you have a SR of 3510, you could end up in a match with an average SR below 3500 and not have hero pools. One other thing to note is that you can still end up in an “Master and Grandmaster” match even if you or other players are in placements.