A trace flag is a special setting in Salesforce that captures detailed information about the execution of Apex code1. It records various events, such as the execution of Apex methods, SOQL queries, and DML operations, and writes them to a log file1. Trace flags can be configured for three types of entities: Apex classes, Apex triggers, and users2. For Apex classes and triggers, trace flags override the default logging levels and enable logging for the specified class or trigger2. For users, trace flags enable logging for all the Apex code executed by that user2. Trace flags can be set up using the Developer Console, the Tooling API, or Setup2. Flows and Visualforce pages are not valid entities for trace flags, as they are not Apex code34. References: 1 TraceFlag | Tooling API | Salesforce Developers1, 2 Debug Log Levels - Salesforce2, 3 Flow | Tooling API | Salesforce Developers3, 4 Visualforce | Tooling API | Salesforce Developers4