# Using the ACU message reference

The topics in this section describe each of the supported ACU message primitives and include the following information:

Information | Description
--- | ---
Purpose | A short description of the purpose of each macro.
Conn_id | Contains the connection ID of the call that the message concerns.
Macro | The macros you can use to specify values for the information elements (IEs) in outgoing Q.931 messages. When the structure (and associated substructures) containing this data reaches the ISDN protocol stack, the stack rearranges the data into several IEs, builds a complete Q.931 message with the IEs, and sends it to the network. Filling order: Some pointer and size macros must be assigned values in sequence so that the IEs in outgoing messages are ordered correctly. If macros must be filled out in order, the filling order is specified in the protocol variants table of the message primitive. For example, when creating an ACU_ALERT_RQ message, the application must call the Acu_alert_rq_a_uui and Acu_alert_rq_uui_size macros before calling the Acu_alert_rq_a_display and Acu_alert_rq_display_size macros. In the protocol variants table, the Acu_alert_rq_a_uui and Acu_alert_rq_uui_size macros are marked Filling order: 1. The Acu_alert_rq_a_display and Acu_alert_rq_display_size macros are marked Filling order: 2.
Protocol variants | The variants under which each macro is supported. Note: Not all macros or macro combinations are supported for every variant. Macros supported under E10 are also supported under AT&T 5E9.
Macro descriptions | A short description of the macro.
Q.931 IE | The Q.931 information element in which the assigned value appears. Note: Some macros do not map directly to any information element, and several macros are not currently used at all. They are included in the reference table for informational purposes only.