Complex sentence: For that comic he was listed in the credits as the "Big-Background Artist", which referred to his role in both penciling and inking the larger and more detailed background designs only on certain select panels and pages, rather than throughout.
Split into simpler sentences: For that comic he was listed in the credits as the "Big-Background Artist". That odd credit referred to his role in both pencilling and inking the larger and more detailed background designs only on select panels and pages, rather than throughout.