The GBA and DS make extensive use of compression in the files.
Probably the most popular compression type is LZSS and there are several variations upon it usually noted by a flag/hexadecimal magic stamp in the initial bytes of compression or just before.
type 10 is the most common and oldest, so called BIOS compatible and as such SWI calls in the BIOS for it. Most LZSS compression tools you will find geared towards GBA and DS stuff use this method.
type 11 appeared some time back and was incompatible with type 10, crystaltile2 was among the first applications to support it.
type 40 according to the NFO at least appeared recently in the DS release of Golden Sun.
It supports both compression and decompression and should support ARM9 binaries and overlays.
Probably the most popular compression type is LZSS and there are several variations upon it usually noted by a flag/hexadecimal magic stamp in the initial bytes of compression or just before.
type 10 is the most common and oldest, so called BIOS compatible and as such SWI calls in the BIOS for it. Most LZSS compression tools you will find geared towards GBA and DS stuff use this method.
type 11 appeared some time back and was incompatible with type 10, crystaltile2 was among the first applications to support it.
type 40 according to the NFO at least appeared recently in the DS release of Golden Sun.
It supports both compression and decompression and should support ARM9 binaries and overlays.
Always touching 
] brb