ESI metrics derive from citation counts to papers published in journals indexed in Science Citation Index Expanded and Social Sciences Citation Index. Journals are assigned to 22 research fields. In ESI, a journal can be assigned to only one field.
The Multidisciplinary field contains journals such as Science and Nature. In an article-level classification, articles from those journals would be assigned to specific fields. This should be taken into account when analyzing the field ranking of an individual scientist, institution, or country/territory.