Mapping a: S -> T be so that any x ε S has one and only one y &#

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mapping a: S --> T be so that any x ε S has one and only one y &#

What makes it necessary for any mapping a: S --> T be so that any x ε S has one and only one y ε T?
 
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That's the defining property of a being a function from S to T.
 


It is not necessary at all. One can talk about mappings that are not functions as well. What is the context of this question?
 


I see...what I was referring to were particular types of mappings (functions), right?
 


It might help if you told us exactly what you mean by "S" and "T"!

If you are referring to any sets S and T and by "mapping" you specifically meant "function", then yes, that is true simply because of the definition of "function".