A slate usually includes some credits, and some technical specs. It can be just a text block on a black background with a big number counting down the seconds. Generally, a 10-second slate will count down from 59:59:50:00 to 59:59:57:00 (I work in a PAL environment, 25fps, non-drop), then have 3 seconds of black, then the first frame of video at 00:00:00:00 exactly.
The few seconds of black are a failsafe from back in the day, in case the tech spliced at the incorrect frame.
A black flash isn’t as bad as flashing a slate to the audience. ????
With the exception of a countdown clock (in seconds) and generally speaking a circle in the background, you can include whatever the broadcast station / production company / post house asks of you, there is no set of rules to what it needs or can not contain.
A good example of info would be something like this:
https://fcpxpert.files.wordpress.com/2015/10/template.png
Some people like doing it a little more fancy:
https://www.madpanic.tv/tips/files/VTCountDownClock_Tutorial.html#.WZGuQXcjGA0
I prefer just having a big number count down and drawing up a circle in the background. That stems from the days of tape and anamorphic video, so the next tech can check if his pixels are being displayed in the right aspect ratio. But, whatever works!