What is this D rating on glass screen protectors?

[X3Us]

Title basically. I'm looking for a screen protector and I keep seeing all these ratings like 2.5D, 3D, 5D, 6D, 10D, 15D. What the heck do they mean? And these ratings seem to be kept seperate from the H rating like 9H which appears on the above D ratings.

Example: https://ibb.co/TBk53Mk

 

[treetopsranch]

Looking up your question on Google seems to indicate that it is a rating indicator. 1 being soft and higher numbers indicate harder material.