Relative alpha colors refer to an origin color, and only change the alpha channel. The meaning of alpha channels is defined in CSS Color 4 § 4.2 Representing Transparency in Colors: the <alpha-value> syntax.
Relative alpha colors provide a direct CSS way to derive a translucent version of an existing color without rewriting its color channels. Authors currently need to duplicate component values or create separate precomputed tokens when they want “the same color, different opacity.” The CSS Color 5 alpha() function preserves the original color components and only changes alpha, which reduces authoring overhead and makes color tokens easier to reuse and maintain.