Expansion Annual Recurring Revenue (ARR) is a critical metric for scaling SaaS businesses, representing the additional revenue generated from existing customers beyond their initial ARR. It focuses on growth through price and volume expansions, such as tier upgrades, price increases, additional seats, and increased usage, excluding new customer acquisitions. Expansion ARR is essential for assessing product value over time, product-market fit, and customer success, and it plays a significant role in metrics like Net Revenue Retention (NRR). Calculating expansion ARR involves summing up revenue increases per customer over a specific period, with strategies to boost it including designing upgrade paths, cross-selling, and implementing price uplift policies. Pitfalls include misclassifying expansion with new revenue and ignoring contraction. Platforms like Orb offer solutions to optimize and report on expansion ARR by simulating pricing strategies and tracking expansion drivers, ensuring SaaS companies can efficiently grow their revenue base.