A custom slug is the user-controlled piece of a short URL. Where an auto-generated slug is chosen randomly by the shortener for uniqueness, a custom slug is typed by the creator and validated against existing records. Most services enforce a minimum length (often three characters) to reduce the risk of accidentally hoarding single-letter paths, rejects reserved names that collide with site routes like `/auth` or `/admin`, and normalizes to lowercase to prevent duplicate registrations differing only in case. Custom slugs are allocated first-come-first-served inside a namespace. The namespace can be global (the slug is unique across the entire host) or scoped per user — 1URL.at uses prefix routes like `/:username/:slug` so each user gets a private namespace alongside their share of the global one. Custom slugs carry tradeoffs: they are memorable and brandable but also scarce, and once issued the creator is responsible for keeping them alive. Because custom slugs are user input, robust shorteners validate them against profanity filters and phishing blocklists and rate-limit creation to prevent bulk squatting.