A keyword followed by a host language variable. A transaction
    handle identifies each instance of a transaction. If you do not
    declare the transaction handle explicitly, Oracle Rdb attaches an
    internal identifier to the transaction.

    In Callable RDO, use !VAL as a marker for host language
    variables.

    You can put parentheses around the host language variable name.

    Normally, you do not need to use this argument. The ability to
    declare a transaction handle is provided for compatibility with
    other database products and future releases of Oracle Rdb.