The GDB database

A GDB database is a grouping of data layers, usually the contents of a single file. It can contain raster layers (channels), vector layers (or segments), lookup tables (LUTs), pseudocolor tables (PCTs), ground control points (GCPs), a projection definition, metadata, and a variety of other auxiliary information.

Each channel of raster data or segment is considered to be a layer in GDB. This includes GCP segments and LUTs, but does not include the projection definition (even though this is a segment in PCIDSK).