The phrase "skirt insert" refers to a piece of fabric or material that is added to a skirt to change its appearance or provide more coverage. Full definition
The exterior package includes new side skirts, a front spoiler, a rear skirt insert, 4 - end pipes, and a new rear spoiler. (topspeed.com)
All models will receive an aerodynamic kit that includes a new front spoiler, chromed grille, rear skirt insert with integrated sports exhaust, rear roof spoiler, and rear spoiler and side skirts with chromed design elements. (topspeed.com)
For fans of carbon fiber, the AMG Exterior Carbon Fiber package adds a dose of the woven grey - black stuff to the front splitter, fender air outlets, side mirror housings, side skirt inserts on GT C models, and the rear diffuser. (automobilemag.com)