Mark toolbar

In customize Toolbars i see an object with an ! called "Mark", seems similar to a Spacer ?

in the Tip it says
"Marks the place in the toolbar where Namespace-specific toolbar buttons are to be displayed"
So my main question is: what does Namespace- specific mean and how does this Mark think work ?

https://www.gpsoft.com.au/help/opus12/index.html#!Documents/Marker.htm