Used to share data like surfaces and alignments across multiple drawings.
You can use the Autodesk Virtual Assistant to request a download link for older software versions that may not appear in your standard list. autocad civil 3d 2015 15 link
Autodesk retired support for Civil 3D 2015 on February 7, 2020 . It no longer receives security updates, bug fixes, or compatibility patches for new Windows versions. Used to share data like surfaces and alignments