real, dimension(2,3) :: B, C, D B = 3.0; C = 4.0 D = sqrt(B**2 + C**2)will produce a matrix D full of 5's. The meaning of the last (array) statement is that of 6 elemental statements
Note that all six elemental statements are, conceptually, performed in parallel (this may or may not be true, depending on the architecture). In contradistinction, a nested pair of DO loops would produce the same result evaluating the elements of D in a predefined order.
In conclusion, array expressions are evaluated element by element, with no particular ordering implied; conceptually, all elemental operations are performed simultaneously.