Guide Layer

Available by right-clicking on a layer in the Timeline or Viewer, then going to the Options section, activating the Guide Layer option means that this layer will be visible in the Viewer, but not in the final render.

This type of layer is useful for adding spatial cues, such as a Grid generator or a PNG file containing rules not to be exceeded, or a color palette from which the color selector can pick colors.

When this option is activated, the eye icon representing the layer's visibility is surrounded by 4 blue lines:

Invisible in a Sub-Composition

When creating a Sub-Composition, or using a Composition as a layer source, the Guide Layers will not be visible in the Viewer of final rendering either.

In the image below, we've unfolded the Layers section of the Sub-Composition to reveal the Guide Layer inside.