Popups are mini topics used to display a small amount of information in a small "popup” window, which is normally displayed when the user clicks on a link. Popups are used both in the help text itself and in applications, where they are referred to as "field-level popups".
Natively, field-level popups are a special Microsoft technology that is only supported in HTML Help CHM files. However, the responsive skins in the Premium Pack 3 add-on for Help+Manual also add field-level popups for WebHelp.
Topics used as popups should be created in the Project Files section so that they do not have TOC entries. For more details on field-level popups see Using Context-Sensitive Help and Context-Sensitive Help and Popups.
You must create popup topics in the Topic Files section, not in the TOC section! You cannot assign the Popup topic class in the TOC.
Context-Sensitive Help & Popups (Reference)