User-Defined Functions (UDFs) allow you to encapsulate reusable, parameterized DAX logic into named functions that can be called across measures, calculated columns, and visual calculations throughout your semantic model. UDFs support both value and expression parameters, enabling you to centralize complex business logic--like tax calculations or currency conversions--so it's written once and maintained in a single place. Functions can be created in DAX Query View or TMDL View and appear as first-class objects under a dedicated "Functions" node in Model Explorer