Nut Retainers have many purposes, but one of their major functions is to provide the ability to tighten a nut without the use of a wrench. Although both perform similar functions, there are two sets of retainers in the VEX Ecosystem: one sized to fit #8-32 Nylock and Hex Nuts, the other sized to fit #8-32 Low Profile Nuts and all Standoffs. The purpose of this article is to explain the difference between Standoff Retainers and Hex Nut Retainers.
This image shows the difference in compatible size between Hex Nut Retainers and Standoff Retainers. Note that while only the 1-Post Retainer is shown, the same holds true for all Hex Nut Retainers and Standoff Retainers.
For Hex Nut Retainers, the maximum width across flats (or the diameter measured from face to face) on a nut is 8.6000mm (the width of #8-32 Nylock and Hex Nuts), while the maximum width across flats for Standoff Retainers is 6.3500mm (the width of #8-32 Low Profile Nuts and all Standoffs).
This means that #8-32 Nylock and Hex Nuts will only fit in Hex Nut Retainers, and that #8-32 Low Profile Nuts and Standoffs will only work effectively with Standoffs Retainers. While you can place #8-32 Low Profile Nuts and Standoffs into Hex Nut Retainers, they will not be self-tightened as they are too small to be held in place.
Below is an easy-to-reference chart that explains which nut/standoff is compatible with each corresponding Nut Retainer. All nuts can be found here, all standoffs can be found here, all Hex Nut Retainers can be found here, and all Standoff Retainers can be found here.