Yes, amber alerts are one class of alert that uses WEA. It's based on the mobile station (phone's) location. The rules used to be written allowing per-tower granularity, but new FCC rules require 0.1 mile accuracy to whatever geo-bounding the author of the alert sets. The bounding can be specified as a circle, polgyon, or by geocode. Geocodes specify a state (or all), county (or all). Geocodes for EAS (the alert system on FM radio) allowed for county subdivisions drawn by the NWS. I don't actually know if those made it into WEA.