You can control the types of compiler warnings that the ActionScript compiler generates in the Compiler Errors panel. When the compiler reports an error, you can double click on the error to navigate to the line of code that caused the error.
Strict Mode causes warnings to be reported as errors, which means that compilation will not succeed if those errors exist.
Warnings Mode causes extra warnings to be reported that are useful for discovering incompatibilities when updating ActionScript 2.0 code to ActionScript 3.0.