Do witnesses of a Will have to know they are witnessing a Will?

‘….each witness either (i) attests and signs the Will or (ii) acknowledges his signature in the presence of the testator (but not necessarily in the presence of any other witness)’

Attestation means the act of attending the execution of a document and bearing witness to its authenticity, by signing one's name to it to affirm that it is genuine. However, the witnesses don’t need to know the contents of the Will.