This study is looking at a medicine called **elenestinib (BLU-263)** to see if it helps people with **indolent systemic mastocytosis (ISM)**, which is a condition where certain cells in the body grow too much and cause problems. The study will compare elenestinib with a placebo (a pill with no medicine) to see which works better. People who take part need to have tried other treatments that didn't work well enough. **Key Points:** - **Length and Parts:** The study has several parts; some participants may take elenestinib in different phases. - **Visits and Treatment:** Participants will receive either elenestinib or a placebo alongside their usual treatment for ISM. Some will switch to open-label elenestinib later. - **Risks and Eligibility:** People with specific health issues, like certain heart problems, or those who had certain cancer treatments recently, may not be eligible. Participants will need to meet specific health criteria and agree to follow study procedures. The goal is to find a better way to manage ISM symptoms.