The Large Daylight Senior is the largest, most durable softbox Chimera makes. It is designed for use with a Fresnel up to 24,000 Watts with a front diameter between 24 and 29 inches.
It utilizes an extremely deep profile of 50 inches. The extra depth of the Daylight Senior (it's about 30% deeper than the Quartz softboxes) allows the fresnel beam to spread out and cover the front face more evenly. This extra depth, combined with a fabric that is even more heat resistant, makes the box safe when used with a 24Klight.
The "Plus" designation means the front face is removable. Chimera makes different strengths of diffusion material for front face applications. The softbox comes with a one stop diffusion cloth, and additional 1/2 and 1/4 grid cloths. Substituting a half or quarter difusion or taking off the interior baffle would result in even more contrast, slightly more light transmission, and less softening of the light.
The real benifit of the large 54x72" size is the softest light possible.