The Composite Texture node is a traditional composite stack for image textures via the Composite Texture Layer node (figure 1). The composite stack builds vertically where the top most layer is on top and only Composite Texture Layer nodes can be connected to the layer inputs on the Composite Texture node (figure 1).
Figure 1: The Composite Texture node is used to mix a RGB Image node with a Checks Texture node
Add/Remove Input - Adds or removes layer inputs where Composite Texture Layer nodes can be connected.
Clamp - Clamps the results of blending each alyer to 0 and 1.