Hit Levels
There are five hit levels in MKX, as defined by Infl's Fighting Game Glossary they are:
Unblockable. Attacks that cannot be Blocked under any circumstance, if their Hitbox connects with your Hurtbox you will get hit no matter what. The most common type of Unblockable Attacks are Throws, which can be rejected by pressiong TH/1+3 at the same time as your opponent, but there are also some Strikes and Projectiles with this property. Unblockables are often very slow and have a huge tell that they are coming so stay on the lookout for them
Overhead. An attack that must be blocked while standing, mostly used in Overhead-Low Mixups, if you try blocking an Overhead Attack while crouching you will be hit
High. An attack that will whiff (miss) on crouching opponents, some of these attacks may still connect if your character is particularly big or close to the opponent but they can still be blocked crouching
Mid. An attack that can be blocked both standing and crouching, most of the roster's attacks will be either Mid or High
Low. An attack that must be blocked crouching, mostly used in the aformentioned Overhead-Low Mixups to find an opening on your opponent's defense and start running your gameplan, if you try blocking a Low Attack while standing you will get hit
Chip Damage
All attack's in MKX will cause chip damage while blocking, both from normal moves and special moves alike. Throw teching will also cause 2% of chip damage as well.
X-Ray Meter Gain
You only gain meter when you attack blocking opponents and perform Special Attacks. This means proactive players inherently also get more damage and escape options, so don't be too passive!