I figured it out. It is used to cover the antenna cables to m.2 Wi-Fi card as illustrated in XPS 8940 manual.  This serves to prevent accidental detachment of antenna from sockets. Slide and place the wireless-card bracket on the wireless card. [IMAGE][IMAGE] [IMAGE]  
This piece of Dell clear plastic bracket included in part list of XPS 8950 and Inspiron 3910 as well as Optiplex 7090 etc.  DP/N PX9DM what is it used for? [IMAGE] [IMAGE]  
What is the use of Dell P/N PX9DM BRKT,FOR M2 WIFI ANTENNA.