Propagation Groups
Propagation modifier works by generating strands on top of other strands. This can create powerful effects such as feathers and vegetation. However, to achieve these effects there have to be a means of procedurally modifying specific groups of strands separately. For example, when creating feathers you will most likely want to set the radius of the feather stem to be larger than the radii of the feather hairs. You might also want to assign them different styling properties.
Ornatrix achieves this by assigning a separate identifier to each group of propagated strands called "Propagation Group". This identifier separates strands within the same hair structure into multiple groups which can then be separately altered down the hair evaluation stack.
Assigning Propagation Groups
Strand Propagation modifier contains two parameters designed to work with propagation groups. "Propagate On" parameter specified which propagation group new strands should be generated on. This is useful if multiple propagation modifiers are applied inside the same modifier stack and you wish to propagate strands on already propagated strands. "Assign Group ID" will specify the propagation group to assign newly generated strands. This value must be unique within the current modifier stack if you want the generated strands to have unique properties.
Using Propagation Groups
Many modifiers in Ornatrix such as Render Settings modifier and Frizz modifier contain a parameter called "Apply to Group". This parameter specifies the unique propagation group to which the modifier will be applied. All strands not matching this group index will not be affected by the modifier.