When I first came to Second Life, most clothing was on "clothing layers." These are textures that are painted directly onto your classic avatar body. (See my earlier post, "Layers and Layers of Layers" )
At first, there were only skin and clothing layers. Over time, new layers were added. First we got tattoos. Then came the "physics" layer that isn't really a clothing layer at all, and is used to make your body jiggle. Finally, the old invisiprims were replaced by alpha mask layers to hide parts of your body under mesh clothing and other clothing or body part attachments.
The big problem was that you could only wear one thing on each layer. One undershirt, one shirt, one jacket. Clothing designers often provided the same top on all three layers, so that it was easier to put together an outfit.
Then Linden Lab made it possible to wear up to five items on a clothing layer by using the ADD command instead of WEAR. This made it possible to stack shirts, for example...but the biggest benefit was that we could add multiple alpha masks...one for your SLink feet, one for your hands, one for your mesh pants, another for your mesh top, and so on.
But people were still running out of layers. I had to compromise some of my outfits because I maxed out my available alpha masks.
Now LL has come to our rescue again. They've implemented the "LayerLimits" functionality in the viewer. I am not sure exactly when this moved from a project viewer to the released viewer, but it's been in place there for at least several months. (Nobody ever TELLS me these things!) It's in the Firestorm viewer too, and probably most of the other popular third party viewers.
LayerLimits removes the five items per clothing layer limitation. Now you can wear up to 60 (!!) clothing layers IN TOTAL, in any combination. This does not affect the "Essential Four" items (shape, skin, eyes, system hair) or the physics layer. Only one of each of these items is allowed. But it does affect tattoos, alpha masks, and the traditional clothing layers such as jacket, pants, gloves, etc.
Woot! Good job, LL.