Burp Suite’s Montoya API exposes a small utility that ranks a collection of HTTP request/response pairs by a custom anomaly metric. The official Javadoc describes the entry point but not the math behind the score.
Each response is summarized into a fixed set of analyzed attributes. Each attribute has a type (what aspect of the response it captures) and an integer value that encodes which “bucket” or variant that response falls into for that aspect. Think of the integer as an opaque label: two responses with the same label for STATUS_CODE are treated as identical for that feature.
The Montoya enum AttributeType (see reference below) lists every dimension that can participate. Together they cover status, important headers, cookies, body fingerprints, HTML structure, word counts, forms, and more. Only attributes that vary across a batch of request and response pairs are used in the score.