From Forum Nokia Wiki

Each application program is run from a shell program, which may be activated by pressing the swap-and-launch application button in the phone. Before running the program, at least two files must exist: an application file having an extension .app, and an application information file having an extension .aif.The application file contains, at a minimum, a unique identifier (UID) to identify the program. Each file in the operating system contains unique identifiers, which consist of three 32-bit values and a checksum.

The Application Information File(AIF) may specify the icon for the application, the application caption in many languages, and capabilities, such as whether the application can be embedded or whether it is shown in the application launcher.

AIF Builder helps to create the required files. It is independent of device family and programming language. AIF Icon Designer helps to create the bitmap for the application icon. The icon must be in a multi-bitmap format (.mbm), which is the specific format required by Symbian OS. Normal bitmaps may be converted into multi-bitmap format by using the bitmap converter tool bmconv.

Note: AIF Builder tool has been deprecated in Symbian OS 9 because the icon format is different.