The unit of curation is different in all of these imaginary cases, because it should be decided to suit the aims, needs and resources of the institution. The only thing to remember is that once in use in documentation and collections management, it should not be changed without careful thought and detailed recording. How material is used Exchanging scientific material has a long tradition. Many important natural science collections were amassed by exchanging specimens from all over the country and beyond. It has always been considered a way of strengthening collections, by increasing the depth of representation of groups or introducing otherwise missing elements, rather than a backdoor way of disposing of unwanted material, and as such should not be viewed in the same suspicious way as deaccessioning.
Scientific research progresses by revisiting the discoveries of others.
This is true in taxonomy as much as in any other discipline. Often this is achieved by lending specimens to other experts or institutions for research. Much natural science material is of low financial value, and there is a lot of it. In many cases it is accepted practice to post material to researchers without detailed preliminary exchange of letters and agreement of terms. In fact, detailed procedures would inhibit research.