Application data (entity instances) stored in a file in the ISO 10303-21 format should conform the the Express schema(s) that is called out in the header of the file. But it practise this is not always the case. The validation capability of Eclipse allows to detect instances that violates the constraints defined in the Express Schemas.
In fact there are two different validation tools available. The basic validation is running within an Express Compiler project and validates a given p21 file against the compiled schema(s) in this project.
The extended functionality is specialized on p21 files containing data according to STEP Application Protocols. Files of this type must contain entity instances according to the AIM respectively MIM of the APs and in this case the file should have the extension ".stp". But the files should also follow the higher level constraints given in the Application Reference Model (ARM) and Mapping Specification of the APs. For the validation of these files JSDAI uses pre-compiled STEP Express schemas of the ARM (in the form of XIM), AIM/MIM and mapping specification.
Detected errors are reported in the Problems view and the p21 file Viewer.