A block is defined in a block definition file, which has a .bdf extension. This XML file describes the functionality of the block and includes its implementation in EPL. With Software AG Designer, you graphically define the block's interface, and Software AG Designer automatically generates all the XML elements of the .bdf file. In addition, Software AG Designer generates skeleton EPL code for the block's behavior. Software AG Designer provides a dedicated editor where you add your custom code and it validates the EPL code you add.

The block definition file actually defines an EPL template. The term "template" is used because the EPL in the .bdf is not complete EPL code. Instead of the actual block name, the .bdf code uses a specially encoded stand-in for the real block name. The real names are automatically generated when the combined scenario and block are converted into full EPL code when they are injected into the correlator.