Minecraft wiki entry for Placed_feature, associated text: The root tag.
 feature: The configured feature.
 placement: A list of placement modifiers. They are applied in order. A modifier takes in a block position and returns a list of block positions. This allows them to filter, transform and generate block positions.
 type: The placement modifier type.
Other parameters of this placement modifier, described below. Note that unlike configured features, these are not wrapped in a config object.