The top type is also frequently used to hold objects of unknown type.

A completely unknown type of civilization must have developed itself here.

So means that the given list contains objects of some unknown type which extends the class.

A mill of unknown type was built c1603 and removed by 1616.

A mill of unknown type was built in 1856.

Finally it remains the author's decision whether he grants the rights to exploit his work via a new, yet unknown type of use or not.

And they are always both set to 1, if you want to disable drives of unknown type.

It used a single jet engine of unknown type for propulsion.

Another mill of unknown type is known to have stood on the Millwall.

The aircraft will be five Boeing 727s and one of another unknown type.