From Version 3.6, user can flag an item in Checklist as needed. Here is a screen-shot of a checklist with some items flagged:
like Show checked only/Show unchecked only in title context menu in view mode, Show flagged only can be invoked from title context menu in view mode as well
Flag all/Unflag All: since flagging item is done in Edit Mode of checklist, Flag all/Unflag All can be invoked from title context menu in edit mode
A popular use case for flagged items will be shopping list, which user may have a more comprehensive shopping list initially. If he/she need to buy some stuff, he/she can flag the item(s) in shopping list. During shopping time, he/she can filter flagged items by Show flagged only menu item in title context menu. After shopping is done, the flagged items can be un-flagged all and start over.
Marker: Flagging item can be used to mark an item in checklist as needed.