There doesn’t appear to be an official Apple explanation for all four box styles. Based on user reports, Aperture’s AF-point display seems to be camera-specific and not always reliable.

For Canon DSLRs, the most consistent interpretation reported is:

• thin white: inactive AF point
• thick red: selected/confirmed focus point
• thin red: active point, but not the primary selected one
• thick white: unclear/less consistently documented

Some users also found that thicker boxes may indicate greater confidence that focus was achieved, but this is not documented by Apple.

Because Apple’s own help only clearly supports the simple case—one red active point and the others white—and because mismatches have been reported for some Canon models, the extra red/white variations may reflect metadata quirks or an Aperture bug rather than something you can fully trust. In short: use the overlay as a rough guide, but don’t assume every box style maps cleanly to what lit up in the camera viewfinder.