Here is interpretation from Dallas BEST head Judge:
I checked the rules and there is no specific guidance or definition to differentiate signs vs posters.
My interpretation is that posters are meant to be mounted and fixed, such as part of the display booth or hung up in the school to promote and advertise the team. Signs are hand held and displayed by spectators in the stands or team cheerleaders and fans. Both signs and posters are graded under the same section of the rubric, so as long as they are part of the “multiple avenues of support - signs, posters, props, costumes, t-shirts” and are “creative and noticeable”, they should get graded appropriately.