Cheese can be made to have a wide range of functional characteristics like melting / non-melting, which various different options for example complete melt [typical pizza cheese], restricted melt [cheese in sausage / meats] and non-melting [cooking cheese].
The technical description for melting is the ability to flow or stretch - this effect depends on the interactions of caseins, the major protein found in milk.
** Refer to the article attached for more information on how the casein network, cheese composition, acidity, proteolysis & buffering can influence the melt & stretch of cheese.