I cannot understand what the Value Relation widget of the Fields menu in QGIS actually does.
For example: I have a point layer with some columns (ID, value1...) and a table (so just a dbf file) with other values (ID, valueA,...). If I open the Fields menu of the point layer I can set some options for the selected column: Layer (set to table.dbf), Key column (set to ID) and Value column (set to valueA).
Now, if I create a new point, it appears in the row of the widget column a dropdown menu where I can choose the valueA of the table.dbf. And that's ok.
But, what is the meaning of the Key column?
Does someone have an explanation for that? Am I missing something?