While exploring array property nodes, I came across this "Is on Block Diagram?" property.
Though we cannot create a property node for a control which doesn't present in block diagram, there is a node to check its presence. Even with control (in my case array) in block diagram, there is no value change in Boolean indicator.
First, it is a scripting node as seen by the blue background on it. That means normal LabVIEW programmers should never need to use it. It might be useful for programmers who are using scripting to programmatically create LabVIEW code.
This node doesn't have to be just for a control, it could be used other items such as constants, function nodes, structures .....
Beyond that, I don't know how useful it is, but NI might have had a use for it and thus created the property and exposed it through scripting.
For your situation, I wouldn't ever worry about it again.
That property is inherited from Generic.
All nodes in the Generic hierarchy get that property. Numeric is in the Generic hierarchy (Generic>GObject>Control), so it gets the property.
Of course, if you know you have a numeric control, you know it's not on the block diagram. But if you make a VI that acts on generic (refs of the "Generic" type) or GObject references , it can be useful to know if the node is on the block diagram or not.