Every time a camera rolls on any high-end production set, a Camera Slate is placed in front of the camera for the first few moments of camera roll in order to label the footage that is about to be gathered by the camera.
When there are multiple cameras on set, there is a common occurance of using one slate to label both or multiple cameras when they roll. The person holding the slate will raise the slate and hold it so multiple cameras can clearly see the labels written on the slate. When the cameras roll, the operators will say "Camera Speed" and the person holding the slate will say "A/B Common Mark" then clap the slate down if there is a separate audio recording device. If audio is being recorded on one or all of the cameras, it is acceptable to leave the clapper arm of the slate closed but the statement "A/B Common Marker" is still spoken aloud. If the second or other cameras are in fixed positions or are behind or to the side of the slate, it may be necessary to clap the slate once for the "A Camera", then turn the slate to face the "B Camera" and the Assistant Camera person will say "Second Sticks for A/B Camera Mark" and clap the slate a second time as it faces the other camera/s.
Potentially, one or more of the camera operators may need to point their camera towards the slate when they begin the roll and will reframe their camera to another area of the set after the slate has been clapped.