[Zavien Doombringer]

Does the Constitution says what form the declaration must take? Or the wording it must contain? NO

Then why was "Resolution of Force" used instead of "Declare War"?

[capitan_refugio]

"Then why was "Resolution of Force" used instead of "Declare War"?" To make the vote politically palatable for some of the Senators and Representative. It's called "plausible deniability" or "political cover." ("You can fool some of the people all of the time, or all of the people some of the time ..." - Lincoln) The resolution, however, had the full effect of a war declaration.