Here is the list of the x$ tables and what they are used for.
The first two letters in the name of the x$ tables means:
KS: Kernel Services
KX: Kernel Execution
K2: Kernel Distributed Transactions
KK: Kernel Compilation
KZ: Kernel Security
KQ: Kernel Query
KA: Kernel Access
KD: Kernel Data
KT: Kernel Transactions
KC: Kernel Cache
KJ: Kernel Locking
KG: Kernel Generic
The complete list of the x$tables can be found here.
KS: Kernel Services
KX: Kernel Execution
K2: Kernel Distributed Transactions
KK: Kernel Compilation
KZ: Kernel Security
KQ: Kernel Query
KA: Kernel Access
KD: Kernel Data
KT: Kernel Transactions
KC: Kernel Cache
KJ: Kernel Locking
KG: Kernel Generic
The complete list of the x$tables can be found here.