Researchers are looking for more ways to treat PAH. In PAH, the blood vessels in the lungs become thick and narrow, which makes it harder for blood to flow. This causes high blood pressure in the lungs and overworks the heart. PAH can make it hard to breathe and be active. Some standard (usual) treatments for PAH can treat symptoms of PAH but do not stop PAH from getting worse. Sotatercept is a study medicine designed to treat PAH. It is a targeted therapy, which is a treatment that works on certain proteins that play a role in causing PAH. This is a long-term follow-up (LTFU) study. People who took part in certain other studies testing sotatercept for PAH may be able to join this study. The goal of this study is to learn about the long-term safety of sotatercept and if people tolerate it when taken with standard PAH treatment over a longer period of time.