Process:
1) Units are deliberately selected to be moved up from A tier into S tier. Out of those, some units are further promoted into S+.
2) Then units are demoted down from A tier into B tier.
Criteria:
1) Units are promoted out of A tier if they offer something that is hard to find in the rest of the unit pool. (Note that this gives bladetome, broadleaf, and razzle dazzle users a disadvantage. While powerful, it's hard for bladetome 1 to distinguish him/herself from bladetome 2. This also favors units who give blessings.) This typically involves some combination of powerful exclusive skills and stats.
2) Out of the units in S, units considered exceptional in terms of utility and/or power even among the rest of S are promoted into S+.
3) Units who really struggle to justify their use at all are demoted down to B.
^ denotes possible promotion candidate
† denotes possible demotion candidate
A tier is kept large to reflect that most units in the game are completely usable with investment. But maybe I'll split A into two parts some time. asdf| Tier |  |
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