Blending modes for layers control how each layer blends with or interacts with layers beneath it. Blending modes for layers in After Effects (formerly referred to as layer modes and sometimes called transfer modes) are identical to blending modes in Adobe Photoshop.
Most blending modes modify only color values of the source layer, not the alpha channel. The Alpha Add blending mode affects the alpha channel of the source layer, and the silhouette and stencil blending modes affect the alpha channels of layers beneath them.
You can’t directly animate blending modes by using keyframes. To change a blending mode at a certain time, split the layer at that time and apply the new blending mode to the part of the layer that continues. You can also use the Compound Arithmetic effect, the results of which are similar to the results of blending modes but can change over time.
Each layer has a blending mode, even if that blending mode is the default Normal blending mode.