Payment optimisation is often framed as an authorisation rate problem, but authorisation is only one component of a larger system. Each Transaction moves through Checkout, Fraud evaluation, authentication, authorisation, clearing and Disputes, with distinct decision points at each stage. These decisions are interdependent: a Fraud model that blocks too aggressively might reduce Dispute rates but also reduce conversion, while an authentication strategy that minimises friction without accounting for risk will increase declines and Disputes.