The Flight Information Service Broadcast (FIS-B), also commonly referred to as ADS-B weather, is continuously broadcast by a network of ground-based towers. Each tower provides a range of aeronautical information. There are four (altitude) tiers: High, Medium, Low, and Surface.
Each tier of tower provides different products at different (Look Ahead) ranges. ADS-B receivers are capable of receiving data from multiple towers at once. What you see in ForeFlight is a compilation of all the towers you are receiving.
Most towers throughout the United States are low-altitude tiered. Low-altitude towers provide less information than medium and high-tier but are able to be received at lower altitudes.
High-altitude towers provide the most information, including the ability to receive METARs for the major airports (Class B & C). If you are receiving a high-tier tower near Seattle, you can get the METAR and TAF for KBOS, but not a smaller airport such as KBED (class D) near Boston.
The table below depicts ADS-B weather products offered by the various types of towers:
ADS-B Product Parameters