Reform 2 feature for parsing nested form submissions. When a nested fragment comes in for a property (e.g. a size hash inside product), the declared populator decides what to do: create a new nested object, find an existing one, skip it, or something custom. Nick Sutterer describes populators as simultaneously the best and worst idea in Reform: they are genuinely a parsing/deserialization concept, but because Ruby/Rails culture rarely talks about parsing explicitly, users overloaded and abused them far beyond their intent. Reform 3 replaces populators with a step-based parsing pipeline built on Trailblazer activities.