You can apply a material to a selection of renderable sub-objects, such as faces in a mesh. In the Material Editor, you can use Assign Material To Selection. You can also drag the material from the Material Editor or the Material/Map Browser to the selected faces. This can create a new Multi/Sub-Object material on the fly.
You can disable drag-and-drop of materials to sub-objects. To do so, go to the Preferences dialog 
General tab, and in the Sub-Materials group, turn off Assign Automatically. This checkbox is on by default.
How the Multi/Sub-Object Material Is Created
The Multi/Sub-Object material is created in one of three ways, depending on what material is already applied to the selected sub-objects:
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No material applied
If the selected faces have no material applied, a new Multi/Sub-Object material is created. The dragged material becomes a sub-material in the new material. If material IDs already exist, they are preserved.
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Existing material applied (other than Multi/Sub-Object material)
A new Multi/Sub-Object material is created and applied to the selected faces. The existing material is moved into the Multi/Sub-Object material and becomes the first sub-material. Unselected faces get material ID #1, the selected faces get material ID #2, and the dragged material becomes part of the Multi/Sub-Object material. Existing material IDs are not preserved.
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Multi/Sub-Object material applied
If the existing Multi/Sub-Object material is already applied more than once in the scene, the material is copied and the new copy is applied to the selected faces.
If the Multi/Sub-Object material is only applied once in the scene, then the existing material is used. The dragged material is added to the existing Multi/Sub-Object material.
If the dragged material already is a part of the Multi/Sub-Object material, then the selected faces receive the corresponding material ID number. If the selected faces all have the same material ID number, and no unselected faces are already using this number, then this number is used and the new material replaces the old sub-material at this ID. Otherwise, a new material ID number is assigned to the faces, and used for the dragged material. In this case, any existing material IDs are preserved.