Yes, the diamond does indicate a filling food.
The bracket is telling you the "set point" value for that food.
Set points are for when you want to eat more of that particular food... usually isn't a larger portion, but you're just supposed to eat the food until you feel satisfied, you don't have to measure the portion, and then you count that food as whatever the bracket says for points.
Set points are kind of confusing, and I don't know anyone at my meeting who ever uses them, but they are a good option when you want to eat a little more of something without measuring it. I'll warn you that set points doesn't mean you can eat to your hearts content. You can't go overboard with the portions and still expect to lose.
I'm fairly new to WW, but my understanding of set points is that they're for use in situations where you really can't measure: parties, restaurants, etc., rather than when you want to eat more. I'd say when I want to eat more is when I need to measure most!
You're supposed to eat a reasonable portion of the food, eat mindfully, and stop eating when you feel full. The set points are higher than the points for a measured serving of the item. The idea is compromise - for the flexibility of being able to eat away from home and enjoy social events, you pay a slightly higher points value.