What is the meaning of "minimal key"? In the definition, it says minimal key is a minimal set of attributes needed to identify the table. Does the "minimal" part mean "minimal number of attributes" or "cannot be eliminated"?

For example:


For a relation R(A, B, C, D, E, F), the closure of A, BC, and DEF are all {A, B, C, D, E, F}.


Are they all minimal keys? Or is only A the minimal key?
They are all minimal keys.

Minimal means there is no proper subset with the same property.