At NLM, classification by publication type takes precedence over classification by subject. For example, an atlas of Liver Neoplasms is classed in WI 17 along with atlases on any other subject covered by the WI schedule for digestive diseases. Other libraries that prefer to shelve material by topic can assign a subject call number.

When both a main heading and a subheading have their own form numbers, generally classify for the subheading. For example, for a work with the subject Education, Nursing $x history, use WY 11.

When both a topical subheading and a publication type are classified by form, classify for the publication type. For example, for a work with the subject Surgery $x history and the publication type Outlines, use WO 18.2

For more information on special cases of form number classification, visit the NLM Policy on Classification.