A configuration map's required maps and an application's prerequisites and subapplications can be specified using configuration expressions. Configuration expressions are strings containing Smalltalk code that evaluates to either true or false. Configuration maps and applications can have any number of configuration expressions associated with required maps and both prerequisites and subapplications, respectively. The development image and each passive image maintain a dictionary of values that can be queried to determine the type and purpose of the image.

The cross development environment extends the notion of configuration expressions as described below. The term "current image" below refers to the image (development or passive) into which configuration maps and applications are being loaded.