Message #do: has aBlock as parameter: What is the 'that' object in DynamicCell

Trying to understand the model:

DynamicZell has

do: that
	behaviour := that.
^ self.

Which type of object is that?

Note: If you check all implementors of the message #do: in the Smalltalk image you find that parameter of #do: is aBlock in nearly all cases in Smalltalk