Compound operator with built-in evaluation rules.
can only be interpreted by default when it appears with or other integral operators.
f x is by default interpreted as Integrate[f, x].
\[DifferentialD] is also used in TraditionalForm to indicate total derivatives.
See The Mathematica Book: Section 1.10.4, Section 1.10.4, Section 3.10.1, Section 3.10.3 and Section 3.10.4.
See also: \[PartialD] , \[CapitalDifferentialD] , \[Delta] .