@Oceana – My husband has ADD, and he has trouble with both selective and sustained attention. Sustained is probably his most problematic, because he is so easily distracted.
He will start out doing a task, but he will be unable to finish it, because something else catches his thoughts or his attention. He jumps from task to task, never completing any of them because something else came up.
It is really hard for him to focus on one thing, so using selective attention is nearly impossible, at least for any length of time. If we are having a conversation in a public place, before I even finish a sentence, his eyes have wandered off to something else in the room, and he often interrupts me with a comment about what he is looking at.
It's hard on him, but it's hard on me, too. I never have his undivided attention, because he doesn't have any!