It is a new partially complete feature with 2.0 and I haven not gotten it documented yet. The possible values are primaryKey, foreignKey, uniqueConstraint, none. The idea is that it may solve the problem of some databases creating indexes on primary keys and foreign keys and constraints and others not. If we have createIndex changes tagged with the reason, we can skip the creation of them if the database will inherently create one.
When I do document it, I am going to mark it as experimental. There is a lot of questions on what to do with how the diff tool should logically work, and it will be the focus of 2.1. The associatedWith field may go away depending on what is decided.