GIF files have a colour palette that goes with it, that contains up to 256 index colours for the image.
A custom palette derived from the actual colours in the image.
A palette containing the exact colours used in the image. Only images with 256 colours or less can use this palette. If there are more than 256 colours, then this becomes an Adaptive Palette.
40 of the colours are reserved for the colours used by the computer's operating system and browser windows. The remaining 216 colours are then used as either an Exact Palette or an Adaptive Palette fashion.
The palette uses the 256 colours defined by the system (Windows/Mac).
A palette of 256 or less shades of grey.
A palette of 256 or less colours that has been custom created.