Spacers allow you to fit a shorter motor into a longer case. They are the size of 1 propellant grain. So, for example, you can fit a 3 grain motor into a 4 grain case using 1 spacer. Or, a 3 grain motor into a 5 grain case using 2 spacers. Do not use more than 2 spacers.
Click here for instructions on how to use the spacers (info is at the end of the document).
The standard spacer will bridge the gap between the 6GXL and 6G cases with 75mm hardware. No XL spacer exists. This is unique to 75mm hardware.