This touches upon the question what objects should be in scope to get a digital specimen identifier and related to that, what we see as a specimen. Over the past two years there has been a lot of discussion on this, for example in the last TDWG conference and in relation to DarwinCore and the MaterialSample class (see for instance: Change term - MaterialSample · Issue #314 · tdwg/dwc · GitHub).
What I get from these discussion is in short that a specimen is a material sample and what people in collections usually refer to as a sample is actually a subsample (usually of a specimen as material sample). I think this is not the whole picture but I will get to that later.
During TDWG2020 we had a survey “What do you think a specimen is”, see for results: Do the "What is a (physical) specimen" test - SurveyMonkey Dashboard. With only 18 responses it is probably not representative for the community but nevertheless it revealed some interesting things: most people seem to agree that every curated object with its own physical identifier in a collection is a specimen. For example if a fish is dissected in a head, tail and fins and these are preserved as individual objects, then there are three specimens. Also most people see an object as a specimen as soon as it has been gathered (sampled). This means that an object should ideally have a digital specimen identifier already in the field (as in this picture: digital extended specimen infosheet - Google Slides), while it gets a physical specimen identifier later, when accessioned in a collection.
This makes the distinction between a material sample and specimen even more fuzzy, because a material sample is the result of a sampling event while a specimen is the result of a curation process. So you should expect the object to be a material sample after the sampling event in the field, and it becoming a specimen after it went through a curation process at the institution. However you could argue that the curation process often already starts in the field with initial preservation of the sample (for example an insect that is put in alcohol after catching it).
There is one category of specimens that is not a material sample though in my opinion and these are specimens that are the result of a recording. For example a sound record or a drawing. You could even see field notebooks or register books as specimens in that they are the results of multiple recordings. T
A thing for discussion then is whether these always need to be physical objects as results of recordings, like a sound tape, or photo on paper, or if these can also be digital objects like digital photographs or models. I think these can be seen as specimens as well, when they go through a curation process. This means they will need to have a physical specimen identifier as well then. Likewise any (sub-)sample should be treated as a specimen, when it went through a curation process (got an identifier and is preserved and stored).
For digital specimen identifiers we have so far been focusing on natural history only, excluding any human related collections such as anthropological collections, archaeology etc. I think we should go for an extendible model where other collections can also use digital specimen identifiers in the future, if there is demand for it.
The more generic we can keep the metadata in a PID record, the more inclusive it can be. However in the discussions around MIDS (Minimum Information of a Digital Specimen we see that the requirements for metadata are already different at a very minimal level, where we probably can distinguish between different needs between preserved specimens, fossil specimens, living specimens, earth sample specimens and recorded specimens. For instance for an earth sample specimen you want to know the material type, but for a preserved, fossil or living specimen this will always be ‘organic matter’. For a record specimen like a drawing you probably want to know the artist and for a fossil specimen you want to know the geological time period. We cannot create schemas for all possible collections at once, so my proposal would be to start with the natural history specimens which have been our main focus up to now.