The ISIC classification is mandatory because it is used to identify the responsible ecoinvent editor for review at the time of submitting the dataset.
The International Standard Industrial Classification is a classification system provided by the United Nations. This system has been implemented in a slightly extended form by the ecoinvent database in version 3 (see the list below for the additional codes in ecoinvent).
Please try to provide a classification using at least 3 digits to better specify your activity. The original classification with explanations is also available at the website of the United Nations.