Navision Customizations is done in C/SIDE.
C/SIDE (Client / Server Integrated Development Environment) is the core of Navision and is the foundation for almost all functionality in Navision. It is made up of 5-7 building blocks called objects (tables, forms, reports, dataports, codeunits, xmlports (introduced in ver. 4), menusuites (introduced in ver. 4)). These objects types are shared throughout Navision, which resolves in a uniform and consistent interface to the application.