In Fire Emblem Heroes, the all players are in some tier, but I can't figure out what determines this. For instance, the first week, I (and I think all players) was in tier 1, the second week I was in tier 2, and this week I've been in tier 3. From what I've found, you only fight players in the same tier as you, but I haven't been able to figure out what determines tier each week.
If it helps anyone figure it out, I've been scoring around 3100 - 3600 in offense each week and 250 - 270 in defense, but have been unranked.