Dt[f, x] gives the total derivative .
Dt[f] gives the total differential .
Dt[f, x, n] gives the multiple derivative .
Dt[f, , , ... ] gives .
Dt[f, , ... , Constants -> , ... ] specifies that the are constants, which have zero total derivative.
Symbols with attribute Constant are taken to be constants, with zero total derivative.
If an object is specified to be a constant, then all functions with that object as a head are also taken to be constants.
All quantities not explicitly specified as constants are assumed to depend on the .
Example: Dt[x y] .
Dt[x y, Constants -> x] .
You can specify total derivatives by assigning values to Dt[f], etc.
See The Mathematica Book: Section 1.5.2 and Section 3.5.2.
See also: D, Derivative.