SBU is Sensitive but Unclassified. It is commonly seen acronym that is used in the civilian side of the house. NMCI/NexGen, I take it is the Navy Marine Corps' NIPR or NIPRnet, a.k.a your non-classified network. In the civilian side of government we have FISMA policies developed by NIST (National Institute of Science and Technology) and that provides a guideline to developing your agency's security framework. I do not know to much about the DIACAP, I do know that it maps to FISMA in some regards.
But it is I think it is possible to have some applications for retirees, public outreach, or better yet recruiting tools in the cloud available to tablets and smartphones (Apps and webapps) . The Army has a two titles in the appstore such as The US Army Survival Guide, ARMY Physical Readiness Training App, and the US ARMY News and Information App. The Marine Corps has quite a few apps as does the Navy Federal Credit Union. Last but not least, the Air Force has an application for Airman Fundamentals! Many of these apps have been developed by DOD contractors such as General Dynamics.
So these might not be internal applications for use by the general employees, but they are being signed off by the DOD as a means for public outreach. Question is --Who is hosting them.....