Hair and Guides
Ornatrix distinguishes between guides and hair. Understanding the difference is essential, because operators are designed to work on one or the other, and the whole grooming workflow is built around it.
Guides
Guides are a relatively small number of strands that define the approximate shape and flow of a groom. They are the control layer you work with directly: you brush them, cut them, and assign data to them. A useful analogy is a control cage in mesh modeling - a coarse, editable structure from which a much denser final result is derived. Because there are far fewer guides than final hairs, editing them is fast and predictable.
Guides also carry extra information that hair does not, such as selection, and the channels and groups you assign. This is why most hand-editing and styling happens at the guide level.
Hair
Hair is the dense, final set of strands, generated by interpolating between the guides. A groom might have a few dozen guides but hundreds of thousands of hairs. You typically generate hair late in the stack, with Hair from Guides Operator, once the guides are shaped the way you want. Styling operators can be applied before or after hair generation, depending on the effect.
Roots
Every strand begins at a root on the surface, which fixes where it grows from and how it is oriented. Roots that are defined relative to the surface deform with the mesh, so hair stays attached as a character moves. This is what keeps a groom locked to its object during animation.
Strand states
Operators declare what kind of strands they accept. Some require guides, some require dense hair, and some accept either, working on the common notion of a strand. Guides and dense hair can both be treated as strands, but a strand cannot be turned back into a guide with its full extra data. In practice this means you generally create and shape guides first, then generate hair from them, rather than the other way around.
Why this matters
When an operator does not behave as expected, the cause is often that it is acting on the wrong kind of input - for example a guide-only operator placed above Hair from Guides Operator, where only dense hair exists. Keeping the guide-then-hair order in mind, and reading each operator's documentation for what it expects, avoids most such surprises. See Objects and the Operator Stack for how ordering works.