What do you understand from this picture?
It’s the switch of my bathroom heater. It has 4 positions: 0, fan, 1 and 2. It’s easy, isn’t it? Even though, last week I had to open it to try to repair it, and I discovered that one of the two small radiators, it has inside, was not working. I tried to repair the damaged one, but finally I gave up. So I drew two black icons next to the switch, an arrow and a cross, to inform that only the position “1” works.
Surprisingly, my flatmate didn’t understand it. It is easy for me: an arrow is marking the correct choice, and a cross is marking the wrong one. But somehow it’s not as easy for other people, who have not a wide knowledge about “User interfaces”. This is a quite interesting question to think about, when we are creating UIs: “is the user going to understand the interface?”
A trivial example. Look at these two boxes. You can guess the first one is a combo box, or drop-down, or something like this. The second one seems a text box with a drag-and-drop element. Can you see it? We decide the second box is drag-and-drop because it has four arrows instead of one arrow (or this triangle representing an arrow head). Honestly, it’s totally ridiculous!
The essence of the question, when you are working on an UI, is to assume nothing. Quite difficult anyway.