When user attempts to download the application, the JAD file is downloaded first.

The purpose of the JAD file is to provide a very small file that allows the software on the phone to determine in advance if the JAR file is appropriate. This process is called "pre-flighting".

The Java environment on the handset reads the attributes in the JAD file to determine whether it should download the JAR file.

For example, in order to make sure that there is enough memory available on the device to download the file, the device checks the size attribute in the JAD file against the memory available on the phone. If there is not enough memory available the user is informed and the download is terminated.