Environment > Distributing IDL Applications > Callable IDL Applications > Preferences for Callable IDL Applications

# Preferences for Callable IDL Applications

IDL’s preference system allows developers, administrators, and individual users to control default values for many aspects of IDL’s environment and configuration. Creators of runtime applications can take advantage of the preference system to customize the environment in which a particular application runs.

See Preferences for Runtime Applications for a discussion of using preferences in the context of a IDL runtime application.

The process of specifying preferences for a Callable IDL application is complicated by the fact that users never launch IDL directly. This means that in order to specify preference values, you must do one of the following:

- Modify the `idl.pref` file in the `resource/pref` subdirectory of the application distribution. This method is only useful if you are distributing an IDL distribution to support your application — you should not modify an existing `idl.pref` file in your end user’s installed IDL distribution.
- Instruct your users to set environment variables that correspond to the preferences you need to specify, or explicitly set the variables yourself in a startup script or batch file.