Nested collections
In general nested collections should be avoided, since the overall goal in the DOMS is to have as few objects as possible.
If nested collections can not be avoided, the organization of these should be embedded, in the object template for the relevant section (descriptive/technical) of metadata.
Thought example
A new collection is created, upon review of the collection it is discovered that there are different physical media of storage, lets say it is reeltape and Long Play records.
This will result in either two disjunct collections or one collection, the obvious choice would be to make one collection and to include the relevant data about the origin of the digital object.
But there are two origins so how should it be done, the correct approach is to make one collection and two templates to accommodate the two different origins.