There is a continuum that spans zero wind/indoor to SUL, UL, Regular and onward. IMHO, it's about being optimized for a specific role. I started flying indoors in the early 90's before light, tapered carbon was introduced. In Oct 1994 I made a Rev 2.5 using first generation SkyShark 2p and Orcon (metalized ripstop mylar.) 1oz ripstop leading edge. Adhesive ripstop corner reinforcements.
I still have the kite, although it's not flyable anymore. As the Orcon aged the ripstop fibers come away from the mylar. Without the ripstop the sail is very frail and prone to tear/puncture. Nice thing about Orcon is that it can be taped with 3M adhesives. Long ago I described this in a KiteLines article.
No bridle. Minimal everything. Flies well on 18-24' of 30# spectra, depending upon the height of the venue. I used it indoors mostly. Only occasionally outdoors.
The trouble with indoor vs outdoor is that outside you're never assured of zero wind. A 3-4 mph gust can come along and surprise you. So something that's extremely light and also fragile might be best indoors only.
I can recall doing some work on calculations for sail loading. That is, the mass of the kite / sail area, to project low wind capability. I don't have recall of the numbers, but there was a table of sport kites where I tried to compare them on theoretical basis.
Also, I found that it was possible to be too light. It takes mass to have inertia. Without enough mass you lose control. The mass of the lines can become disproportional in their impact on it's behavior during slack line moves.
Lacking for mass you're forced to compensate with added velocity. Sustaining velocity in zero wind can be hard on the body.
