This layer implements some functionality common to simulators and utility programs. Its design is intended to be extensible in order to adapt the interface to new simulators or special demands. Many semantic rules and checks are implemented in the application layer, making the adaption of existing simulators to PIF easier, and ensuring interoperability in the VISTA framework. High-level functionality and automatically invoked data-manipulation services are provided to relieve TCAD tools from routine work like coordinate transformations and unit conversions. The routines of the application layer implement geometry-manipulating as well as attribute-manipulating functions, because we think that both aspects are closely related in a TCAD environment. This fact is expressed in the uniform data representation of geometries and attributes on geometrical objects in PIF.