Curiously, they used the APEX symbol , normally used for luminance value.

An apparent synonym for luminance value, from APEX.

Luminance information can be gleaned from the chromatic information; e.g. the chromatic value implies a certain minimum for the luminance value.

It is meant as a method for correcting photometric measurements of luminance values impinging on the human eye by scaling them by the effective pupil size.

This system stores a luminance value with two chrominance values, corresponding approximately to the amounts of blue and red in the color.

Normal color interpretation mode provides choice of colors from a 128 color palette (16 colors with 8 luminance values for each)

The low nybble of the register specifies one of 16 luminance values ($00, $01, $02... to $0F).

In the normal color interpretation mode the lowest bit is not significant and only 8 luminance values are available ($00, $02, $04, $06, $08, $0A, $0C, $0E).

In GTIA's color-only and luminance-only color interpretation modes the full 16 bits of luminance values are available allowing a palette of 256 colors.

The luminance value of the background is forced to 0.