This page describes the Plantronics C315-M headset model 204440-01 (Newegg item 0TH-000R-00589). This is the so-called "Microsoft" model. Nobody can explain what the -M suffix really means. It's a minor USB feature used with Microsoft Lynx software, but it's not strictly needed.
Read review of the nearly identical Blackwire C315 model 204440-02
Read review of the nearly identical Blackwire C315 model 204440-02