Though their 2018 Supporter's Shield is still up in the air, Atlanta has clinched a Champion's League spot.
By rule they get the spot that would normally go to the 2017 MLS Cup winner. Since that was a Canadian Team (Toronto) it goes to the team with the most aggregate "points" over 2017 and 2018. Red Bull NY has clinched second place on that list, which is important because they get the final Champion's League bid if the 2018 MLS Cup winner is them, a Canadian Team, or a US team that has already earned a bid (Sporting KC (17 US Open Cup), Houston (18 US Open Cup) or Atlanta (see above).
Houston is already eliminated from the Playoffs, so it won't be them. The other eliminated (e) teams cannot qualify for the 2019 Champions League except Toronto, who has already earned Canada's lone bid (Voyageurs Cup).