When is a fence considered light weight and how do you chose if you need a heavy or a light fence?
There is no specific definition of what 'lightweight' would mean for a fence. But, in general, one could probably argue that a fence that is mostly decorative is lightweight. A 3' high vinyl post and rail fence would be lightweight.

An 8' wood privacy fence would not.

You might be able to use these, but I'd want them sunk into concrete so that you have a solid enough base to with stand the horizontal loads the fence in the picture would be exposed to due to wind.