When you import a reference, maya imports a node of the type "reference", which has a diamond icon and you will find them by the end of the outliner. This node is a Reference Node, and thus the RN at the end of its naming. If you have imported that reference more than once (or you have a node already with the expected Reference Node name), the following reference nodes will have a number at the end of them to avoid name collision.
Apart from that, Maya imports the nodes within the referenced maya file, those will have also a little diamon too.
Maya reference system can be a bit chaotic, specislly when it comes to the spooky Reference Edits.