What does the red flag mean?

[StR]

Kostya,

I received a message that is marked by Aquamail with a red flag just to the left of the sender's name. What triggers that red flag?

I am suspecting that it is be one of these two lines in the message headers:
   X-Priority: 1
   X-MSMail-Priority: High
but I would like to know for sure.

Thank you!

[Kostya Vasilyev]

X-Priority: 1