The "SL-ToolNumber" is a property name while you're looking at the values in that drop down list. You can only edit and add/remove the list for the property name but not the value list.
But for property tab builder you can create you own drop down list for selection.
The summary information dialog is the manual way to set custom (part and assembly models, and drawings) and configuration specific (for part and assembly models. The custom properties tab is a way to build a simple user interface with the configuration specific tab. Your SL-ToolNumber property is in the Custom tab which (unless it has changed in SW2017) cannot be accessed from the Custom Properties Tab.