[GeeTwo]

Essentially, as long as it either fits within some team's 30 pound withholding allowance or is built in the pits (or official machine shop) during the competition from COTS parts and doesn't break any other rules (e.g. having mercury), it's OK; see sentences I emphasized in R18 below. As noted above, the receiving robot will have to be reinspected (and more specifically re-weighed).

Originally Posted by R18

At an Event, Teams may have access to a WITHHOLDING ALLOWANCE of FABRICATED ITEMS, not bagged per R15, to be used to repair and/or upgrade their ROBOT. The WITHHOLDING ALLOWANCE is a static set of items that shall not exceed 30 lbs. With permission from another Team, Teams may also have access to FABRCATED ITEMS that are part of that other Team’s WITHOLDING ALLOWANCE to repair and/or upgrade their ROBOT. The WITHHOLDING ALLOWANCE may only be brought into the Venue when the Team initially loads in at the Event.
Items made at an Event do not count towards this weight limit.

There is no restriction on the quantity of COTS items or items which do not meet the definitions of COTS or FABRICATED ITEMS (e.g. raw materials) that may be accessed by a Team at an Event.
For Teams attending 2-Day Events, these FABRICATED ITEMS may be used during the Robot Access Period and/or brought to the Event, but the total weight may not exceed 30 lbs. FABRICATED ITEMS constructed during the Robot Access Period and bagged with the ROBOT are exempt from this limit.

Edit:

Originally Posted by FRC_498

Our alliance 2122 & 125 decided to add some rails and fabric to our robot for the finals at Arizona North regional to try and block the vision tracking of our opponent. Its didn't work well but we did have to get re inspected and re weighed before returning to the field.

Here's another example of not meeting another rule. If the rails and fabric were intended to block vision tracking, they would have violated R9, particularly as amplified in Q953. If they were intended to block shots and just happened to block vision, that would be OK.