It is proper Netiquette to use a descriptive subject line. Do tell what it's about in the subject.
A descriptive subject line for an email increases the chances of the message being read by the recipient because they know what the message is about.
When a person gets an email message in their inbox the senders address and subject of the message appear. Recipients read messages more often when they know what they are about.
Use a descriptive subject line to ensure it gets read because we delete messages based on subject.