It is different from other similar commands on Menu and directly works not on the selected face but on clicked one.
You can choose [Single face] or [Joined faces] for selection method.
Single face
It attaches the material to the clicked or dragged faces.
By clicking with Shift key on a face, the assigned material can be retrieved as the current material.
Before clicking
After clicking
Joined faces
It attaches the current material to all faces joined to the clicked face.
Before clicking
After clicking
Brush
It attaches the current material to the faces over which the cursor overlaps while dragging.
The cursor size can be changed by [Radius]. And, it can be also changed by dradding with Alt+dragging on the 3D view.
Segmentation (EX edition only)
By simply specifying two or more materials on some faces, the materials can be automatically assigned so that the curved parts become boundaries.
Click or drag the faces to specify the base point to be filled while switching the current material in the Material panel, and turn on [Preview] after assignment to see the filled result.
Specify by dragging
[Preview] ON
If modifications are necessary, drag on the face further to specify the base point while toggling [Preview] on and off.
Changing the [Edge detection] will preferentially separate sharp edges between faces. Adjusting the value may give better results, especially with noisy data
Finally, press the [Apply] button to complete the material assignment to the faces.