4.1 Operational and structural understanding
All types of representation can call forth 
operational and structural understanding (Sfard, 1991), 
which are also known as procedural and conceptual 
understanding (Hiebert and Carpenter, 1992).  Thus, 
numerical, algebraic and graphical forms can be 
interpreted in terms of the operations that underpin them, 
or as entities, which have properties and can be operated 
on. For example, f (x) = 3x can be interpreted as 
implying a multiplication operation, which produces 
function values; or might be recognised as representing a 
linear function, which can be differentiated.