1.15. Descriptions should be on topic and relevant to the specific item. Users may not include anything in the description for branding purposes, such as user IDs and commission information. Referencing "sets" (for example: "set 1/3") is not allowed.
This rule for Custom apparel is totally fair I feel- up to the point of not being allowed to reference that your apparel is part of a set. Why can't users make sets and let people know that there are more pieces to match it available? I personally think it would be helpful if an artist is allowed to tell me this information, or give me a keyword to search for to find the rest of their related apparel.