If, after the opinion is issued, the national court asks the examining division to appear before it, the court is informed that the EPO is willing to send one member of the division provided that costs are paid and on the understanding that this member will be required only to answer questions on the technical opinion given and will not be required to give an opinion on additional matters unless notice in writing of these additional matters is given to the examining division at least one month before the appearance before the court.
Source: http://www.epo.org/law-practice/legal-texts/html/guidelines/e/e_xiii_5_6.htm
Date retrieved: 17 May 2021