{ 1 } - the negative form (what "real" contrasts with) is more basic.
{ 2 } - it's used to distinguish narrower and wider senses of a term.
{ 3 } - it's the most general in a series of related words.
{ 4 } - something could be a real X but not a real Y.

Other words in the same group include "natural," "proper," "genuine," "true," and "authentic." These contrast with words like "artificial," "fake," "false," and "toy."

Since "real" is so general, it's often clearer to ask a more specific question. So instead of asking "Is this real hamburger?" it may be clearer to ask "Is this beef hamburger (instead of turkey hamburger)?" -- or "Is this meat hamburger (instead of a soybean product)?" -- or "Is this edible hamburger (instead of being a wax model)?"