When you customize the Slate Material Editor interface, changes persist between sessions of 3ds Max.
Material/Map Browser interface files have a MPL file name extension; for example,
our_mental_ray_layout.mpl
. You can save and restore Material/Map Browser layouts by using Material/Map Browser Options menu 
Additional Options Load Layout or Save Layout As. This submenu also has a Reset Material/Map Browser option that restores the default layout and appearance of the Browser.
Slate Material Editor layout is saved in the file docking.cfg .
These files are saved in the application data folder; for example, \users\<username>\appdata\local\autodesk\3dsmax\2016 - 64bit\enu\sme\ .
Note: The rendering in a Preview window does not remain valid between sessions of 3ds Max, or when you open a new scene.