HP L 5 manual Byte 1 Pixel Encoding Mode, Value Pixel Encoding Restrictions Mode

Byte 1 (Pixel Encoding Mode)

This byte designates the format of any subsequent raster images. The range of the value is zero to three. All other values for this field are ignored.

Value	Pixel Encoding	Restrictions
	Mode
0	Indexed by Plane	Bits/Index can only be 1, 2, 3, 4, 5, 6,
		7, or 8. Bit/Components 1, 2, and 3
		are ignored
1	Indexed by Pixel	Bits/Index can only be 1, 2, 4, or 8.
		Bit/Components 1, 2, and 3 are
		ignored
2	Direct by Plane	Bits/Components 1, 2, and 3 must
		be 1
3	Direct by Pixel	Bits/Components 1, 2, and 3 must
		be 8

	The number of bits per index determines the size of the palette
	created by this command. In the case of the Indexed by Plane mode
	the number of planes needed to represent the index is also
	determined by the number of bits per index. Therefore, if a 256 entry
	palette is needed, then the bits per index is set to eight since
	28 = 256. If the Indexed by Plane mode is chosen, at most eight
	planes are needed to represent each row of data.
	The recommended pixel encoding mode is Direct by Pixel, since this
	gives the most efficient raster processing. However, using this mode
	means that delta row compression should be used since it exploits
	redundancy between rows. Other PCL compression modes exploit
	redundancy within a row. With Direct by Pixel the redundancy from
	pixel to pixel in a row is masked by the differences at the byte level
	within the pixel, that is, the differences between the red, green, and
	blue bytes within the pixel.
Note	Raster data in Index by Plane or Direct by Plane modes cannot be
	compressed using raster compression mode 5.
	You need one plane or one bit/pixel for each power of two colors in the
	palette. For example, a 256-color palette requires 8 planes or
	8 bits/pixel (28 = 256).

EN Using Color Modes 2-7