Milestone has NOT Open Sourced its VMS
IFSEC has reported that Milestone has open-sourced their VMS. However, Milestone has confirmed that this is false and that they were referring to Milestone as being 'open platform', not 'open-source.'
While it is disturbing that a major publication / show does not understand the difference between 'open source' and 'open platform', it is worthwhile to briefly explain how they differ.
Open Source
Open source refers to software that typically allows for use and redistribution, without having to pay for a license. Moreover, generally, the source code of the software is available for review, modification, and extension as well. Examples of open source include the Firefox browser, Linux distributions like Ubuntu, server software like Apache, etc. In video surveillance, the only well known open source VMS is Zoneminder, though it is rarely used outside of hobbyists or technically inclined users (also, it's last stable release was August 2011).
Open Platform
By contrast, the 'open' part for 'open platform' typically refers to integrating or extending software using open standards. An open platform need not, and typically does not, allow modification or redistribution of its internal source code. Moreover, generally, open platforms charge for access to their platform.
Milestone has been marketing 'open platform' for many years, in contrast to what it calls 'proprietary jail'. Examples Milestone offers are integration with IP cameras, video analytics, PoS software, access control, storage hardware, etc.
'Open platform' is inherently a fuzzy claim that has gotten more confusing as surveillance standards, like ONVIF, have emerged. For instance, Honeywell recently claimed to be the most open VMS in the market. Most VMSes now are fairly open to 3rd party IP cameras. However, the depth of openness still varies greatly, as VMSes cannot simply rely on standards to integrate with many systems, as they require integration with proprietary APIs/SDKs.
All this noted, Milestone's recent promotions are more focused on its expanded Linux support, with its Arcus platform release. That said, Milestone is far from the first to support both Windows and Linux.