When the CP preprocessor encounters a COPY statement it marks the source lines as documented in the topic Handling COPY Statements, and reads through the copybook. It locates the copybooks using the same pathnames and file extensions as the Compiler would.
If a copybook cannot be located, the CP preprocessor comments out the COPY statement and generates appropriate error messages. If there is an error in parsing the statement, the preprocessor ignores the statement and carries on parsing tokens from the point of the error. If other preprocessors receive an unmodified COPY statement they can assume that the CP preprocessor was unable to handle it; they should leave it and let the Compiler generate an error message.