Before you type urgent into that email, pause and reflect. Is this truly an emergency? Is this something that can wait? Could this have been planned out?
Urgency is detrimental to everyone. The individual submitting the request is almost certainly frustrated. Whether they waited until the last minute or a fire broke out, The person who gets the alert may have to stop working on something that could be time-sensitive to look into something that might not be. Almost every situation results in an increase in stress. If the ticket is not urgent and can wait, you are cutting the line in front of someone who may have been working on a time-sensitive ticket.
Here are a couple determining factors on whether a request is urgent:
These requests jump the line and get prioritized as soon as possible. These requests are also handled in such a matter as to put out the fire enough to allow a continuance of work.
A word of caution though. It's becoming too common. Someone sends an email with the subject "urgent," "ASAP, or any other form of "drop what you're doing," only to discover that the situation was not actually that urgent. The vast majority of "urgent" communications are not urgent. What you consider an emergency may not be an emergency in the true sense of the word. The issue is that as terms are abused, they lose their meaning.
During normal business hours, I'm assuming an EoD response time for all incoming emails as I write this. This is also written from the perspective of a technical person.
If the situation can wait for a typical response, it is unlikely that it requires an urgent or asap response.
TLDR: If you have an emergency, don't hesitate to use Urgent or ASAP. However, Don't tell someone something is urgent and expect them to drop everything unless it's an actual emergency. If it can be planned ahead, it's not urgent. Don't confuse needing priority with telling someone to drop everything and skip the line. Being the one who cried wolf hurts everyone.