#15- Explain accumulation.

In general, each SP-S treatment adds a layer of laminate to the cuticle, smoothing it out a little more each time, up until about the 3rd-4th treatment.  At that point, the hair is probably as straight and long lasting as it will ever get. Some hair(mine, for example) goes really straight with one treatment. Other clients start really loving their hair only after 3-4 treatments. I've got clients who are mostly battling fuzz, who are now able to go every 6 months between treatments because they've had at least 3 treatments. 

In most cases, the first SP-S treatment(only 1 layer of laminate) lasts the shortest amount of time, and doesn't go the straightest. As each new layer is added, the effects last longer and go smoother, typically. Sometimes, this is absolutely not true and I've seen a handful of clients who's hair resists taking on any accumulation.
Clients can affect the amount of accumulation by the timing of their treatments. For more accumulation, SP-S appointments should be scheduled closer together. For less accumulation, a client needs to let most of the SP-S wear off between appointments. Some clients really don't want their hair too straight, so they might schedule their appointments 5-8 months, or more, apart.
The entire head is done with every SP-S treatment, so the root area will always ONLY have one layer. The ends could have multiple layers.
Let's say a client starts getting SP-S. On their first appointment, the entire head is coated. So there is one layer on the hair. Then, after 4 months, the client comes in again. (There will be @2inches of curly new hair at the root with no SP-S, and there will be the rest of the hair with a little SP-S still on the hair from the first appointment.) The entire head is treated again. That means that the first 2 inches at the scalp now have one layer and the rest now has two layers. The double layered parts will be smoother than the root area. So, the next time that client comes in, the entire head is done again. The first @2 inches of new hair will have one coating, the next 2 inches will have 2 layers, and the rest will have 3 layers......etc. If a person has shoulder length hair, they'll probably only have 4-5 coatings at their ends, at most. People with mid back length hair(like mine) will have many many more coatings at the ends.
If a client does not want too much accumulation in their ends, they could choose to only do their roots, and not layer over the ends every time.
Clients come in sometimes with other brands of SP-S already on their hair and that is no problem for accumulation. All brands pile on top of each other just fine.