To convert content so it becomes hidden text in Word, set a unique value of @outputclass for the material in DITA XML, and assign the following property to the resulting format:
[WordStyles]
; Hide makes the content Word hidden text.
ParaFmt=Hide
When you assign the Hide property to a paragraph format, all text in that format remains hidden in Word unless the user chooses to view hidden text. This setting is meant for Word output only; the Hide property is ignored in WinHelp.
If you really want the content not to be included at all in Word, see §15.4.4 Omitting content from RTF output.