Yea I always wondered what that means.  Is it always 5 lines?
Sorry, it's not just five, it actually looks like this: 
IIII IIII IIII I 
So it was actually sixteen. I was just describing the base unit of five, since I couldn't think of how else to describe it. And yes, it's always the same number of lines. it's part of the texture of the shield, so all imperial shields have it.