Two critical parameters for Advanced Receiver Autonomous Integrity Monitoring (ARAIM) are the probability of satellite fault, Psat, and the probability of constellation fault, Pconst. A satellite fault is one whose root cause is only capable of affecting a single satellite; while a constellation fault has a root cause that is capable of affecting more than one satellite at the same time. This paper provides more specific definitions for each of these fault types. We describe how performance commitments supplied by Constellation Service Providers (CSPs) are used to complete the fault definitions and to estimate their probability of occurrence. Providing a precise definition of what constitutes a fault is essential so that all observers are able to agree on whether or not one has occurred. This paper is intended to lead to a framework for an open and transparent system for determining these parameters. This framework is intended to be the basis for an internationally agreed upon process for determining the fault rates that may be safely used for ARAIM.