According to MalwareBytes Forum is means 'Possibly Unwanted Modification' and yes is mostly likely that.
I would ask the question there if I were you as they would know best.
It's odd that you got that though as I use MBAM fairly frequently as a backup security scanner and have never seen that one.
I just ran Malwarebytes and it picked up the PUM also on my system. I've had my Windows Firewall purposely disabled for years now while just running McAfee Internet Security. So this is as you said Ex_Brit, possibly an unwanted change to one's system.
Well to be more precise, in the Windows Control Panel>>> Security Center...I have it disabled to notify me about my Windows Firewall being disabled.
So this is the reason Malwarebytes is alerting the user>>>just in case you did not make the change and some malware did.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Security Center\UPDATESDISABLENOTIFY (PUM.Disabled.SecurityCenter) -> Bad: (1) Good: (0) -> Quarantined and deleted successfully.
I just ran malwarebytes and it picked these up also. The only thing I can think is that I just reinstalled McAfee (Cox Security Suite) version because my real time scanning and virus protection kept turning itself off. I was told by the tech that when the program updated itself some components did not install. The one I had trouble with was McShield.exe. McAfee seems to be working fine right now and I am wondering if McAfee set these parameters because it alerts you itself when there are problems and doesn't want windows security suite doing so. From what I have read on other forums Norton does this
PUM - mean possibly unwanted modification & as I stated earlier and if you need more information you should ask on the Malwarebytes Forum but I feel, I could be wrong, that MBAM is objecting to the fact that security suites such as McAfee take over Windows SecurityCenter.
In which case it should be disregarded.