Mandatory Fields

If you have not filled in a mandatory field, or used an invalid value (such as a negative number where you're not allowed to enter a negative number), then a little red round icon will show up next to that field.

The red error icons won't disappear until you click the OK button again. That is, they won't disappear as soon as you enter a value - they disappear when you click "Apply" or "OK".

Sometimes though, you might have actually entered a value into a field, clicked "OK", and yet the red error icon is still shown.

In this case there is most likely validation performed on the value you entered. i.e. it's not just a case of putting any value in the field - there might be some restriction on what that value can be (such as negative numbers not being allowed).

If it's not obvious what's wrong with what you entered into that field, you can place your mouse over the red icon and it will tell you.