Composite is a node that can group others nodes, and all nodes are treated as one part alone when it gets to interact with otheres nodes, that aren't on composite.
This node type can pass tranparency to all its children. (It does better job related to opacity).

On Composite, these proprieties that can be edited:

• Translation: x, y, z
• Lock to root node
• Rotation: x, y, z
• Scale: x, y
• Snap to pixel
• Sorting

• Texture offset

• Multiply RGB
• Screen RGB

You can check blending list here.

• Thereshold
• Mask Sources
  • Mode mask
  • Mode dodge
  • Delete