HMC supports command line (ssh) as well as web (https) user interfaces and REST API.
zSeries mainframes prior to the IBM System z9 used a modified version of OS/2 with custom software to provide the interface.
The web-based nature means that there is no longer a difference between local console access and remote access, which means a remote user potentially has full control if authorized, allowing more flexibility for locating systems within data centers.
IBM refers to the new HMC as a "closed platform" which does not allow the user to install software or access the command line interface to increase security and stability.
The HMC is also firewalled by default with a minimal number of open ports for remote access.