There are two different snap functionalities: 

The first is for a typical Relux scene, and the second for imported CAD drawings. 

Here we will consider the typical Relux scene (for CAD snap mode please see capture "CAD Import“).

The icons in the toolbar are:

switch the view of the snap grid on and off.

raster properties (lower right corner): change the grid size, snap into the center, etc.

snap into the center (you’ll find it in the menu raster properties).

The center snap option in the raster properties menu is very helpful for recessed mounted luminaires in certain ceiling systems. 

You can use the above-mentioned icon for this. To change the grid size, for instance, open the “Raster properties” menu.

Centered grid on the floor plan.