It can be a fine line, but we make a distinction between display purposes and costuming/cosplay.
Sure a helmet can be used for costuming but for many, many members, the helmets are used mainly for display purposes. The costuming forum has evolved into more of a separate section for those interested in active costuming and cosplay.
Doing research on an item, even though part of a costume, still fits logically in the prop forum. So many items can be worn as part of a costume that it would be next to impossible to draw a clear distinction between what belongs exclusively in each forum. It is our feeling that, for the most part, the members can decide on the purpose of their thread and therefore determine the best place for it.
Crossover between those two forums was and is inevitable and expected.