Why is PhysicalThing.additionalType recommended rather than mandatory

I see that PhysicalThing.additionalType is shown as recommended [1] whereas
BiologicalEntity.biologicalType [2] was mandatory.  What is the reason for
this change?  I thought that this was one for the most critical properties
(since most entity types will not have their own Bioschemas subclass).

[1] http://bioschemas.org/bsc_specs/PhysicalEntity/specification/
[2]
https://docs.google.com/document/d/1XASuESIHU3bi1aXMxQS5-rCOQX0ugjMNkh68VF4co4Q

-- Justin