This study is checking how well a drug called **AZD0901** works compared to other treatments for people with advanced or metastatic **gastric (stomach)** or **gastroesophageal junction (GEJ)** cancer that shows a specific protein called **Claudin18.2 (CLDN18.2)**. The trial is **Phase III**, meaning it’s in the later stages of testing. **Gastric adenocarcinoma** is a type of cancer in the stomach, and **metastatic** means it has spread to other parts of the body. This study is global, meaning it takes place in multiple countries. Researchers will compare AZD0901 to other treatments chosen by doctors. They will also evaluate a test called the **Ventana CLDN18.2 assay**, which helps identify patients who might benefit from AZD0901. **Key points:** - The study involves multiple visits and assessments over time. - There is a focus on safety and effectiveness of AZD0901. - Participants need to meet specific health criteria to join. Eligible participants must have a stomach or GEJ cancer with CLDN18.2 and have tried at least one prior treatment. They must be able to give consent and meet certain health standards.