Layer
Layering tokens
Layering tokens are explicit tokens used to manually map the layering model onto components. Layering tokens come in predefined sets that coordinate with the different layer levels. For each layer that a component needs to lives on, a separate component variation must be built using the tokens from that layer set.
There are four layers within a theme: base layer, layer 01, layer 02, and layer 03. Layers stack one on top of the other in a set order. Each step in UI color (excluding interaction colors) is another layer and will require the use of a different set of layering tokens.
Follow link below for more information about layer tokens and how they work.