Assign Material to Selection applies the material in the active sample slot to the currently selected object or objects in the scene. At the same time, the sample slot becomes hot.
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(Material Editor): Compact > (Assign Material to Selection)
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(Slate Material Editor: Toolbar > (Assign Material to Selection).)
If you apply a mapped material to a parametric object whose Generate Mapping Coords option is off, 3ds Max automatically turns on mapping coordinates at render time. In addition, if you apply a mapped material with Show Shaded/Realistic Map In Viewport active to a parametric object, that object's Generate Mapping Coords option is turned on if necessary.
The Show Map In Viewport flag is saved with individual materials, so when you drag mapped materials from the modeless Material/Map Browser onto objects in your scene, the mapping appears in the viewports.
The Undo command works for material assignment.
Warning: When you apply a material to an object or selection, that material becomes a hot material. When you change the material's properties, the scene immediately updates to reflect those changes. Any object with that material will change its appearance, not just the objects in the current selection. When a material is hot, its sample slot is displayed with white corner brackets.
To make a material no longer hot so it doesn't change the current scene, click 
(Make Material Copy).