What is the difference between zbabu2 and zbabu3? The composition includes
zbabu3; it is not zbabu3 (meaning that zbabu3 does not encode the
structure, ratios, or process of production of the soap, only its
ingredients). So, any used chemical ingredient is a possible zbabu3. I
would say that any of these could act as a 'source' for the soap,
especially since it is the chemical properties which technically physically
identify a soap, not what produced these properties (although the former
can, as with other things, give a good hint as to the identity of the
latter). On the other hand, any material produced by the source (which is
the important bit), which I could see being the source itself, is an
ingredient.

There are two distinguishing details in my current understanding. First,
the source could actually just be something like "pig fat/lard" or "a
Ugandan shea tree". These (especially the second example) are are not
really ingredients but the locations from which some of the ingredients
originate. In a sense, they stand in as short-hand for the actually
important information about the properties of the soap (which can be tested
directly). For example, a Ugandan shea tree source hints that there is a
high oleic acid content in the butter produced (which would be an
ingredient in a shea butter soap). Second, perhaps not all of the parts of
a material which acts as a source (such as lard) are really used (or
intended to be used) or even saponifiable. These may have been discarded
during the saponification.