DirectionalDiff(A, B_)

The definition of directional derivative

Directional derivative in spherical coordinates

Directional derivative of a vector function

Note that, when the vector which defines the direction (the second argument) is projected over one coordinate system, the function being differentiated is expected to be expressed using the same coordinate system; otherwise an error interruption happens and a corresponding message is displayed

For this example, correct input could be

Identify, Nabla, operations, Physics, Physics conventions, Physics examples, Student[MultivariateCalculus][DirectionalDerivative], tensor/directional_diff, VectorCalculus[DirectionalDiff]., Vectors, Vectors[`.`]