Explanation/Reference:
Explanation:

Values:
OFF
When any of DB_BLOCK_CHECKING, DB_BLOCK_CHECKSUM, or DB_LOST_WRITE_PROTECT are explicitly set, no changes are made.
DATA_ONLY
DATA_AND_INDEX
DB_BLOCK_CHECKING will be set to FULL.
DB_LOST_WRITE_PROTECT will be set to TYPICAL.
DB_BLOCK_CHECKSUM will be set to FULL.

Values:
OFF or FALSE
No block checking is performed for blocks in user tablespaces. However, semantic block checking for SYSTEM tablespace blocks is always turned on.
LOW
Basic block header checks are performed after block contents change in memory (for example, after UPDATE or INSERT statements, on-disk reads, or inter-instance block transfers in Oracle RAC).
MEDIUM
All LOW checks and full semantic checks are performed for all objects except indexes (whose contents can be reconstructed by a drop+rebuild on encountering a corruption).
FULL or TRUE
All LOW and MEDIUM checks and full semantic checks are performed for all objects.