The section below that I have highlighted in red is important when working with Filters and/or Scripts. I have already encountered the "Grayed Out" options in some of exploring.
Layer type
The layer type is determined by the image type (see previous section) and the presence or absence of an alpha channel. These are the possible layer types:
RGB
RGBA
Gray
GrayA
Indexed
IndexedA
The main reason this matters is that most filters (in the Filters menu) only accept a subset of layer types, and appear grayed out in the menu if the active layer does not have an acceptable type. Often you can rectify this either by changing the mode of the image or by adding or removing an alpha channel.
