In realized metaphor, one meaning is represented under the expression of another, as when a courageous person is called a lion. A recognizable shared quality must connect the two meanings.
In representational metaphor, a whole situation or expression is transferred to a similar situation. Its quality depends on the fit between the two complete configurations, not simply on the sounds or dictionary meanings of isolated words.
The common test is whether the selected resemblance gives the audience a coherent and purposeful understanding or instead demands strained interpretation.