A dress protocol is established by a Dominant as the way a submissive is to present themselves for any specific situation. These can be basic grooming habits to very precise details to what to wear to dinner or bed. A lot of times these are the first rules established in a relationship. These can start out as directions for when and what to keep shaved, how to care for your hair, or if you can wear panties or not.
The first rule I had was that I was not allowed to cut my hair. My Dominant likes my hair a certain length and I could only get it trimmed with permission, but never cut more than a certain amount. My hairstylist has adjusted to the change and will ask me now how much I can cut off safely before my boyfriend gets upset. The one to follow directly after that was shaving rules.
Now each Dominant has preferences for clothing and grooming. These are very outward symbols of ownership when followed and adhered to. Dominants can use these protocols to build body image or cause humiliation or embarrassment. It could very well just be a Dominant's preference.
In the case of a party, for instance, there may be a protocol that you must attend in fetish wear and that in order to get in you have to dress a certain way. This is intentional; to keep nosey vanilla people out and to strengthen the sexual and sensual vibe in the party venue. You may very well have clothing and grooming requirements for your private play too. If your Dominant requires you to have an enema and to paint your nails red, this is part of your play protocol. This can take on very different meanings for each couple.
Below is a short list of the dress rules and protocols that I can come up with. I welcome you to include your own in the comments.