From PCS

There are pre-defined error codes in TAF-TSI, but PCS also has its own error codes. The mapping between the two data sets would be theoretically possible up to a certain limit but with a lot of compromises, which would lower the quality of the error information. To bypass this problem, PCS delivers its error codes in TAF format according to the following error code list.

To PCS 

It might happen that PCS accepts a request from the Applicants, but one of the IMs has a problem with that and normally they would react with an ErrorMessage. As the dossier is Path Elaboration phase, the proposed behaviour in such a case would be the following. When an IM cannot accept a path request, it shall send an ErrorMessage to PCS. With that PCS will set the light (acceptance indicator) of the IM to red and place the error to the reason for the red light. It’s then up to the Applicants how they would like to proceed. They can either place a brand new request or they can withdraw the existing dossier.