The presence of spellware is normally obvous to magical detection - even if the function is not. This spellware masks one other piece of spellware you are using, making it very hard to detect. Standard magic scans always fail - only a sustained effort over several minutes can succeed.
Spellware is only concealed if it is not actively used. Activating the magical effect of any piece of spellware breaks the mask - though the spellware itself is not revealed, only the effect.
Spellware concealed with Magic Mask is generally also made unobtrusive by hiding it under your skin. For certain systems, such as Golem Arms, this is impossible, and the Magic Mask is largely pointless.
One shtick of Magic Mask will hide three Spellware systems. Another schtick will hide all but three spellware systems. Three schticks hides all your spellware.