What is the difference between "@attr = ..." and "attr := ..." on class attributes?

Question

The text book for my class warns not to mix up @attr = ... and attr := ... when working with class attributes. It does not, however, explain what the difference is. I know that when working with cells, MyCell := ... changes the contents of the cell, but I have never seen @something = ....

Answer 1

C={NewCell 4}
@C=3
{Browse @C}

The code above outputs nothing, because @C=3 is the same as

{Cell.access C $}=3

and as the function {Cell.access C $} return 4, it is the same as

4=3

To assign a new value to a Cell, you must use `:='