What does +ffff stand for in this DateTime format ddd, d MMM yyyy HH:mm:ss +ffff

I have run into some code where it does something like this:

SomeDateTimeObject.ToUniversalTime()
                                .ToString("ddd, d MMM yyyy HH:mm:ss +ffff", CultureInfo.InvariantCulture),

Now I am confused what +ffff stands for in this. Also, I would like to get the millisecond part of the datetime along with hours/minutes and seconds. what is the format for that?