A "marker place" is a point on a waterway that is used to make sure that maps are drawn properly and also to improve the accuracy with which distances are calculated. They usually mark bends in the waterway, otherwise there is nothing significant there.
All marker places have a name that consists of an '!' followed by the 4 character code that is their internal CanalPlanAC place ID - because they don't really exist they can't have real names.
You shouldn't normally ever see these marker places, but they are of use when editing the data. You can turn their display on and off from the Options menu (go to the "Exclusions" tab, select "Types of places" and they are the top row).