Grid objects let you bypass the home grid and work on separately defined grids to create and position objects. You can use as many grid objects as you like, each serving as a custom construction plane with its own grid settings.
Above: Inactive grid object in a scene
Below: Activated grid object
When creating an object, you can also create a temporary "AutoGrid" grid object that's tangent to an existing object surface at the point you click. To do this, turn on the check box in the Object Type rollout. You can save this grid by holding the Alt key before you click.
Like other objects created in 3ds Max, standard grid objects are placed on the grid of the current viewport. By default, this is a plane of the home grid, but can also be another activated grid object.