CDS Configuration Schema - What is the purpose of 'key element' keyword?

Hi,

I'm reading the developer guide inside HANA Studio and it's not clear the purpose of 'key element' keyword when defining entities.

Of course 'key' must be used to define a table key but I didn't get what is the difference added when 'element' is used after it.

In other words, when defining an entity using CDS, what is the difference of:

entity Entity1 {

    key Fstr:       String( 5000 );
};

and

entity Entity2 {

    key element Fstr:       String( 5000 );
};

?

Thanks,