Remember, every FLEX System module is one-directional.  This "opposite" module is always Front-facing, so it will zip on the the Front surfact of any module.  The idea is that if someone wanted to, they could put a FLAT CAP onto both the front AND the rear of a configuration, thus giving a nice slim look to the bag.

The FLEX FLAT CAP FRONT zips to the Front Surface of all FLEX Center Sections and also directly onto a Rear End Cap (PCR, FCR, PCR).  

You can tell the two FLAT CAPS apart from each other by noticing the Zipper Slider.  This part, the FLAT CAP FRONT, does not have a zipper slider or a zipper pull.  The opposite part, the FLAT CAP REAR, has both.  And the root of the zipper is on the opposite side of each module.  Other than that, these two FLAT CAP modules are identical.

The FLAT CAP FRONT module offers one flat pocket on the outside, and one flat half-height slip pocket on the inner surface of the module. The primary outer pocket is accessed from the top and has a cover flap that is secured with Velcro.

There is a pair of one-third-height panels on the outer-most surface of the module.  The upper panel has the unique ability to tuck and untuck the lower panel. When untucked, the opening allows the telescoping tubes of a suitcase or folding cart to pass through and the panel serves to secure your bag to those tubes. When the upper panel is tucked back into the lower panel, the combination of both panels work together to serve as a thin two-thirds-height slip pocket.

There is also a flat half-height slip pocket on the inner surface of the module.  It has a cover flap secured with a spot of Velcro.  Here you can stow an important document or two, or any small flat item that you'd like to keep out of harm's way.