I'll take a stab without being 100% sure.

If the pedal was designed to take the Look delta cleat both red and black will fit. The Red allows for some degree of heel movement, while the Black has less tolerance before clipping out. From my above 2 pics there are different pedal models both saying ARC. Soooo..., my guess is that ARC is an acronym used by Look across several models.

There were Looks and those who licensed the design, then there were other companies with a similar but competing design. I think Shimano did not license and therefore compatibility of cleats is not guaranteed. ?Am I correct in assuming the trailing edge of the Look had extensions on the outside portion, whereas, SPD-SL is a solid 'lip' across the entire trailing edge?