One of the most powerful features of D-STAR is the ability to route transmissions based on the callsign of the destination. This allows you to communicate with a specific station, without knowing what repeater they are on. Unfortunately, this may not work with people using hotspots or repeaters with this feature disabled.
On Icom radios, there is often a dedicated [RX->CS] button. On Kenwood HT's the [ENT] button is used. After a Gateway station is heard, pressing and holding this button will allow you to set the TO field in your radio to their callsign. When you transmit, your transmission will be routed from your gateway repeater to theirs, based on the last gateway they used.
Pressing the RX→CS button again after the conversation is over typically returns the radio to the previous settings.
This can be useful if you are scanning several repeaters and didn't catch which repeater or module they were on, but still wish to respond. It's also handy for reaching out to friends who may be traveling. The users of both your gateway, and the remote gateway will hear your response - regardless of whether or not your friend is listening. Also, be aware that you may be interrupting a conversation on the other repeater, so be polite and keep your transmission short.
It's also important to note the difference between local CQ and gateway CQ while using gateway repeaters or reflectors. If you make a habit of using the Gateway CQ setting, your responses will be shared with any Gateway linked repeaters or reflectors.
On a modern Icom radio, this can easily be set in DV mode by selecting Reflector > Use Reflector in the TO field. If you open the Call Sign (CS) menu which reveals the underlying UR, R1, R2, and MY settings from older radios, you'll see that this sets the UR field to CQCQCQ and the R2 field to the local repeater's gateway. For local CQ the R2 field is not used.
Simply put, Gateway CQ sends to everyone connected via the Gateway, and Local CQ sends to everyone on the local repeater - but users linked via the gateway cannot hear you. So, while it's not the default CQ setting, it's best to make a habit of using Gateway CQ so anyone you hear can hear you, then reserve Local CQ for when you need to do a quick sidebar with someone on the local repeater without sending it to everyone on the Gateway.
If using routing features, it's a good idea to explain how they are hearing you such as adding "via callsign routing" to the end of your transmission, this should cue knowledgable uers that they'll need to use the gateway to respond to you. Otherwise they could be responding to everyone on the local repeater, but you won't hear them.
You should also be careful using callsign routing while linked to reflectors. If you hear someone on a reflector, and use callsign routing to respond, the response will not be heard on the reflector; only on the Gateway repeaters used by you and the station you are calling. If they're using a hotspot, they may not hear you at all.
Experienced D-STAR users often dismiss the feature altogether - but used properly it is very convenient, but due to the popularity of Local CQ in the US and the popularity of hotspots, it won't always help.
There is a similar original routing technique that enables users to send transmissions to a specific repeater and node. Callsign routing is often the easiest way to respond to a call sent via this method.