D-STAR radios use a set of call sign fields to decide where a call goes. This guide explains them using Icom's official wording, and flags the points where third-party guides disagree or Australian details couldn't be confirmed.
1The call sign fields
MYCALL: your own call sign.
RPT1: the repeater you're transmitting into (the uplink).
RPT2: where the call goes out: the same repeater's downlink, or its gateway.
URCALL: the destination: CQCQCQ for a general call, or a routing target.
2A local call
Icom's guidance for a call without the internet gateway: "Set the uplink repeater call sign in RPT1 and the downlink repeater call sign in [RPT2]. When making a CQ call, set "CQCQCQ" for the desired station call sign." (Icom)
3A call through the gateway
With the internet gateway, Icom says: "Set the uplink repeater call sign in RPT1 and the gateway call sign in RPT2. The gateway repeater has "G" setting for the 8th-digit. When making a CQ call, set "I" plus downlink repeater call sign at the desired station call sign."
Icom also describes call sign routing: "Call another station directly, not using a repeater. The call is automatically routed to the repeater destination the called user is in."
Check this against your radio. Icom's introduction uses "I" plus the repeater call sign, but many third-party guides use a "/" prefix instead (for example / followed by the repeater call sign). The current Icom radio manuals weren't checked for this guide, so follow your radio's manual or your local repeater's notes.
4Reflectors
Repeaters running the dplus software can link to a reflector, such as Icom's example "REF0010C". The commands to link, unlink or query a reflector vary with the gateway software and aren't covered by Icom's introduction, so check your repeater sysop's notes rather than copying commands from overseas guides.
5Registering for the gateway
Gateway routing depends on registration: Icom says the repeater's call sign and IP address must be registered to the gateway server. Users typically also need to register before their calls are routed. The Australian registration procedure couldn't be confirmed for this guide, so ask your local D-STAR repeater's operators or club how registration works in your area.
An F-series call sign may cause problems: ACMA no longer issues them because they're "incompatible with some digital modes" (ACMA call sign policy).