A host or substitution variable, used to communicate with a
    calling (host) program. A host language program declares a host
    language variable and then assigns database values to it. Host
    language variables are not used in RDO.

    You can also use host language variables as names to represent
    databases and database elements. These names are called handles.

1  –  database_handle

    A variable name declared as a longword in a host language
    program. You can use the database handle in several statements
    and clauses to identify a database.

2  –  transaction_handle

    A host language variable you associate with a transaction. If
    you do not declare the transaction handle explicitly, Oracle Rdb
    attaches an internal identifier to the transaction.

    The ability to declare a transaction handle is provided for
    compatibility with other database products and future releases
    of Oracle Rdb.

3  –  request_handle

    A host language variable you associate with a compiled request.