Here's a quick tip on viewport visibility and one more variation on the difference between off and freeze.

The difference between off or freeze for viewports that are not clipped is very simple, they both will make only the viewport boundary disappear visually, not the objects being shown inside the boundary.  For viewports that have been clipped, with the VPCLIP command, the difference is that off will make only the viewport boundary disappear visually and freeze will make the entire viewport and the objects displayed in the boundary disappear visually.