Product managers often err by defining customer segments too broadly, which can lead to weak product-market fit, higher churn rates, and inefficient resource allocation. To avoid this, it's recommended to start with a broad customer segment and narrow it down to a specific "beachhead segment" for testing product and business model hypotheses. This approach is exemplified by Facebook's exclusive launch at Harvard and Tesla's strategic focus from high-end sports cars to mass-market vehicles. The process involves understanding your total addressable market and narrowing it to a beachhead segment, supported by evidence from market research and customer interviews. Emphasizing the importance of targeted customer segments, the text also illustrates how a focused approach can improve product-market fit, retention, internal alignment, scalability, messaging, and resource allocation. Understanding customer segmentation is the foundation for developing a business model canvas, which outlines how to create, deliver, and capture value. Examples from Facebook and Tesla demonstrate the effectiveness of scaling from one segment to another, aligning vision with product and business strategies.