This counts HTTP requests the gateway blocked for exceeding the configured maximum URL length. Overly long URLs are a classic buffer-overflow or exploit-delivery technique against web servers, so a rise here suggests someone is probing for or exploiting a URL-parsing vulnerability, though it can occasionally also catch a legitimately misbehaving application generating unusually long query strings. For example, a request carrying a 9,000-character URL targeting a login page getting blocked and counted here.