Oracle lock mechanism

Row Share (RS)
This lock, also called a subshare table lock (SS), indicates then the transaction holding the lock on the table have locked Rows in the table and intends to update them. A row share lock is the least restrictive mode of table lock, offering the highest degree of concurrency for a table.
Row Exclusive Table Lock (RX)
This lock, also called a subexclusive table lock (SX), generally indicates, the transaction holding the lock has Updat Ed table rows or issued SELECT ... For UPDATE. An SX lock allows other transactions to query, insert, UPDATE, delete, or lock rows concurrently in the same table. Therefore, SX locks allow multiple transactions to obtain simultaneous SX and Subshare table locks for the same table.
Share Table Lock (S)
A share table lock held by a transaction allows other transactions to query the table (without using SELECT ... For UPDATE), but updates is allowed only if a single transaction holds the share table lock. Because multiple transactions a share table lock concurrently, holding this lock are not sufficient to ensure A transaction can modify the table.
Share Row Exclusive Table Lock (SRX)
This lock, also called a share-subexclusive table lock (SSX), was more restrictive than a share table lock. Only one transaction at a time can acquire a SSX lock on a given table. An SSX lock held by a transaction allows other transactions to query the table (except for SELECT ... For update) and not to update the table.
Exclusive Table Lock (X)
This lock was the most restrictive, prohibiting and transactions from performing any type of DML statement or placing any Type of lock on the table.

Oracle lock mechanism