Some users may find the +/- variants fit their machine's alignment marks and materials better than others. Luckily, these are quick to print and easy to swap out! I like to keep spares around as well. For ~$0.50 for a set of 2x5sizes, why not?
For sizing reference, the part pictured on the cutter above is the baseline/standard size guide. The variants equate to tight and loose fits on the 2 existing alignment marks.
Remixed from Allan Porter's replacement mat guide: https://www.thingiverse.com/thing:5244445?fbclid=IwAR1YQJ2_ftIxAzspSRUW-Lgi9EaTcjFgj9dSGpjiEDHqN1pR4pzEuCl4vDI
The author hasn't provided the model origin yet.