What is the difference and why should I worry about this?
Hi Adam- NURBS surfaces are always 4 sided, topologically, making some sort of rectangle, albeit often a distorted one. There are two directions of control points, forming a grid. In order to make arbitrary edge shapes, surfaces often need to be trimmed. This process does not remove anything in the underlying four sided surface definition, what it does it introduce curves onto the surface that Rhino then understands to be trim curves and it knows to show you only the part of the surface that is inside the trims.
Try this: make a sphere, draw a line across it more or less throught the middle, and trim half the sphere away with the line. Then, turn on control points for the sphere- see all those 'extra' points? They are from the underlying surface.
You don't need to worry about this exactly, but it is useful to be aware of the condition- for example, you can Untrim to revert to the larger underlying surface, you can ShrinkTrimmedSrf to eliminate as much of the underlying surface as possible and still retain the exact same trimmed face. Matching surfaces (MatchSrf) for attaining various levels of continuity does not work on trimmed edges (traimmed target edge is OK) etc.