You can have two enabled masks per drawing: one in model space and one in paper space.
If you create more than one mask in model space or paper space, the most recently created mask is enabled; all other masks in that space are disabled automatically.
The mask boundary is still visible in the drawing, but the images that cross the mask boundary are not clipped. You can edit the mask boundary when the mask is disabled, but the affected images are not clipped until the mask is enabled again.
If you used the Hide Image(s) option for the mask, then the images that were affected by the mask are shown when you disable the mask.
If you used the Unload Image(s) option for the mask, then the images are reloaded when you disable the mask.
The images are clipped and the mask properties control the display of the images that are outside the mask boundary.
Only one mask in model space or paper space can be enabled. If you create more than one mask, the most recently created mask is enabled automatically; all other masks in that space are disabled. If you manually enable a mask, then all other masks are disabled automatically.