With the user RH option, the application program specifies the RH in the SEND RPL or in a buffer-list entry. This user RH field has the same format as the SNA RH. The ISTRH DSECT, described in Control block formats and DSECTs, can be used as a map for this field.

The USERRH option can be used when sending any function-management-data or data-flow-control request or response. It can be used either with or without the LMPEO and BUFFLST options. See Table 1 for the location of the initial RH. If the buffer list option is used, each buffer-list entry that defines the beginning of an RU (or group of RUs created by an LMPEO splitting operation) must contain an RH field.

With the user-RH option, VTAM® does not enforce particular settings of RH indicators related to data flow control and FM data. However, VTAM does control the settings of RH indicators related to transmission services, session control, and network control; and also prevents the setting of those indicators reserved by SNA, except RH byte 2, bit 3, the request change direction indicator. Table 1 lists the RH indicators to which the user RH option is applicable.

The RH for a request or response inbound to the application program is made available in the RECEIVE RPL or in the read-only RPL of the RESP, DFASY, SCIP, and NSEXIT exit routines.