Support Element or Support Element Application, referred as an OS/2 software , allows to administrate some functions of some IBM mainframes like the Multiprise 3000 and also on the IBM z890.
According to Wikipedia. "The Support Element is the most direct and lowest level way to access a mainframe. It circumvents even the Hardware Management Console and the operating system running on the mainframe. The HMC is a PC connected to the mainframe and emulates the Support Element. All preceding zSeries mainframes used a modified version of OS/2 with custom software to provide the interface. System z9's HMC no longer uses OS/2, but instead uses a modified version of Linux with an OS/2 lookalike interface to ease transition as well as a new interface. Unlike the previous HMC application on OS/2, the new HMC is web-based which means that even local access is done via a web browser." 
- The Corestore Mainframe: Starting the S/390 part 1 - powering it up. Running Support Element 1.4.4 on OS/2
- The Corestore Mainframe: Starting the S/390 part 2 - loading from tape