What you are showing us in your excellent photos is the portion of the MFT/3 miter fence with can be used under the guide rail.
This is part of the cunning Festool design, as explained in the MFT/3 instructions.
Based on all of your questions, you would really profit from attending the Introduction to Festool End User Training class offered frequently at both Festool USA Headquarters in Lebanon, IN and in Henderson (near Las Vegas) NV.
There are many tips and tricks to getting the most out of the MFT/3 system which are most effectively learned through the coaching in an End User class. Perhaps your Festool dealer could put you in contact with someone located near you who is experienced using the MFT/3