The fm-indexterm element represents an indexterm element as a FrameMaker marker. If the indexterm to fm-indexterm option is enabled in the Options dialog, indexterm elements and any index-related child elements are converted into the FrameMaker index marker syntax and added to an fm-indexterm element.
In order to support the indexterm to fm-indexterm conversion, the Topic and Book structure applications must have the fm-indexterm element defined. This is a clone of the indexterm element, but is defined as a Marker object rather than a Container. All general rules that allow the indexterm element should also allow the fm-indexterm element. The default value of the fm-indexterm element’s class attribute should be “- topic/indexterm fmx/fm-indexterm”. The fm-indexterm marker only supports the conversion of indexterm elements that have the following child elements: indexterm, index-see, index-see-also, and index-sort-as. If your DITA files use other child elements you should disable this optoin.
No rules are needed to properly process the indexterm or fm-indexterm elements. In fact, if you’re migrating an older structure application you should remove the indexterm rule, it is no longer needed.
If the indexterm to fm-indexterm option is disabled, indexterm elements will open as simple container elements.