The simplest way to explain Dharmadhātu is that it is all of Reality, but conceived as all its elements
(as a single category) rather than as a whole.
For example, a cake might be the "body" (kaya) but the slices are the elements (dhātu).
In the case of the Dharmakaya (the Truth-Body, the All) then the elements would be things like matter, phenomena, concepts, etc, and are therefore, together, called Dharmadhātu (Truth-element)
All the slices of the cake are absolutely identical
to the cake, but are given a different name, because they are conceived as slices rather than as a cake.Tu-shun
splits the Dharmadhātu, which is a single category, up into its various constituent elements.