On what kind of martian soil or rock will the Perseverance rover drop its helicopter?

The image below is cropped from the high resolution version of NASA's Mars Helicopter and Perseverance Rover PIA23824 and shows the helicopter in it's stowed position on the underside of the rover next to one of the rover's wheels.

The caption says:

The Mars Helicopter, visible in lower center of the image, was attached to the belly of NASA's Perseverance rover at Kennedy Space Center on April 6, 2020. The helicopter will be deployed onto the Martian surface about two-and-a-half months after Perseverance lands.

For more information about the mission, go to https://mars.nasa.gov/mars2020/.

Question: What are the constraints on the soil or regolith at the "drop point" where the helicopter will be allowed to fall to the ground and I assume after the rover drives a safe distance away, will have to take off from? Can the soil be too coarse or too fine? Or will they look for a clean, flat rock surface instead?

click for full size or visit the NASA page linked above