Readability matters for social platforms. Usernames like x9q2k are technically valid but poor for social interaction. Platforms like LinkedIn encourage real-name based usernames. Gaming platforms are more permissive but still discourage random character strings.
Uniqueness policies vary by platform. Some require globally unique usernames (Twitter). Others allow duplicate usernames with numeric discriminators (Discord: username#1234). Some scope uniqueness to organizations or communities.
Prohibited content goes beyond reservation. Offensive language, hate speech, and adult content terms should be blocked regardless of category. These are naming rules rather than reservation rules — they are about content appropriateness rather than impersonation prevention.
Guidance improves signup completion. Show users why their chosen username was rejected. Suggest alternatives. Explain the rules inline rather than after submission. Good naming rules are as much about UX as policy.