[Taylor]

Section 8, Rule 15:

<R15> ROBOTS must use one of the two FLAGS provided at the event
queuing location to display their ALLIANCE color (red or blue). The
FLAG must be held in a flag holder mounted on the ROBOT. The
only acceptable flag holder is a 12 inch long piece of 1/2” ID
Schedule 40 PVC tube. The tube must be a single, contiguous
piece, must be capped with a commercial PVC pipe cap cemented at
one end. The tube can not have any “lightening holes” or other
modifications other than mounting holes (paint or other decorative
surface finishes are permitted). The flag holder must be permanently
mounted to the ROBOT such that when the ROBOT is in any
PLAYING CONFIGURATION and the FLAG is in the flag holder, the
FLAG is approximately vertical and the top of the FLAG is a
minimum of 18 inches higher than any point on the central mass or
structure of the ROBOT (note: ROBOT arms, appendages, and
extensions may temporarily intrude into the space above the top of
the ROBOT and exceed this height of the FLAG – this a normal part
of game play, and will not be considered a violation of this rule). The intent of this rule is to
insure that the FLAG must be easily seen from any side during the entire match play.

There's also a pretty picture in the manual.