Collapsed graph inside of function can make return node, but it will never deleted

Normally, if you make collapsed nodes graph in Event graph, there would be no option for place ‘return node’
but If you make collapsed node inside of some function, you can see there is ‘return node’ for place node option,
and if you place it, you cannot delete it.
if I press this placed return node, it shows like its sharing parameters with inputs and outputs nodes.
If I add any parameters to inputs or outputs, it will effect on these nodes, but I cannot see it inside of return node.