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Once a session is established between the SSCP and a PU, the SSCP can issue requests to establish a session between itself and any active LU associated with the PU. This type of session, called an SSCP-LU session, enables SNA requests to flow back and forth between the LU and the SSCP. These requests pertain mainly to LU-LU session establishment and termination. For application program LUs, VTAM® establishes the SSCP-LU session with the application program when the application program opens its ACB. See Opening the program for details on opening an ACB. The SSCP-LU session with the application program ends when closing its ACB. See Closing the program for details on closing an ACB. For other LUs (for example, those associated with cluster controllers), VTAM usually establishes SSCP-LU sessions because of a command issued by the VTAM operator.