Sumptuary Law is a list of rules controlling what people of varying classes were and were not allowed to wear, as fit their social station. Medieval sumptary laws varied and tended to become more elaborate as the period progressed. Dressing above one's station was a criminal offence, and an assault on the rigid social order of feudalism.
There is some evidence to suggest that the sumptuary laws were frequently, if surreptiously, broken.
Sumptuary Law in the SCA
Generally speaking, what the SCA regulates is armorial display. In short, one should not wear or display armory or regalia to which one is not entitled. The most common examples of this are the emblems of the various orders and awards. For example, only members of the (hyopthetical) "Order of the Bouncing Jakalope" should wear a medallion displaying the registered arms of the order (Argent, a jakalope rampant upon a cross, sable.)
There are a few other items whose use is restricted by hallowed ancient custom, primarily the restriction to the Chivalry of a plain white belt or baldric. Custom also tends to prohibit any use of a laurel wreath or a pelican. Such broad restrictions are not actually enshrined in law, but most people avoid using those symbols for fear of causing confusion or being seen as presumptious.
It should be noted that no belt colors other than white are in any way restricted. Red belts are used most places to mark squires, but many non-squires also wear red or reddish belts. Some kingdoms have customs governing belt colors for apprentices and proteges, but those customs vary a great deal. (See InterKingdom Anthropology.)