Here I think I found a partial answer:
So, from the article, I understand that (in Australia) a Deed of Trust can specify different classes of units with varying rights attached to them.
Then the article goes on saying:
As in the case of companies, there are various classes of units which can be issued and rights which pertain to these. These may include (but are not limited to):
- Discretionary units – which entitle the holder to an entitlement to the income and/or the capital of the Unit Trust. The Trustee may determine to exercise a discretion to make a distribution to such a unitholder in a similar way to a discretionary trust. These units may also be called Special Units and a Unit Trust with discretionary units may also be referred to as a Hybrid Trust in that it is partly a discretionary trust and a Unit Trust;
So, "discretionary units" already give a great deal of flexibility in the way income is distributed to unitholders... and on top of that, different classes of units could still be defined.
At least, this is my understanding... and based on Australian law only...
Still hope to get a better answer from someone more knowledgeable than me :)