The target services the macaroon is allowed to access is represented as a caveat. The caveat key is
services, while the value consists of a comma-separated list of target services. Each target service is composed of a two-tuple consisting of the service name and its tier. Tiers are service specific and must start from 0, which serves as the base tier. If a specific service tier has its capabilities and/or constraints updated, there needs to be a way to detect when a macaroon of the same tier with the now outdated capabilities and/or constraints is being used. By committing to the service tier, it is possible to detect such cases and seamlessly upgrade the stale macaroon (assuming it is valid) by revoking it and minting a new macaroon with the newer capabilities and/or constraints.