What are "Rare research subjects" ("Kleine Fächer")? I've noticed a funding agency offer a program for funding "rare subjects" ("Kleine Fächer" in German).

I'm pretty sure my own research doesn't count, but:


Is this concept / category of research subjects used more generally or is it just that funding body's invention?
What are the criteria typically used in judging a subject to be "rare"? After all, almost everyone studies subjects which, at some level of specificity, nobody else is studying (so it's "rare" to study it).
From the detailed description of the call:


  The so-called "rare subjects" (Kleine Fächer) often represent a comprehensive, significant and future-oriented canon of knowledge that is highly relevant for interdisciplinarity, internationality, and innovation.
  […]
  At the same time, these subjects are often characterized by precarious structures within universities.
  Above all, the human resources in these subjects are often scarce; there is a lack of structural sustainability for research and higher education.
  Moreover, they are particularly vulnerable to changing orientations in higher education policy, but also to socio-political change.
  
  […]
  
  The assessment will be based upon a comparative
  weighting of the following criteria:
  
  
  Does the proposed strategic concept focus on a “rare subject“? Is it about precarious knowledge?
  […]
  


So, the main criterion is that the subject is so small that it is “endangered” to die due to budget cuts and similar.

On the website of the call you can find a link to a conference on the subject, which in turn links to Portal Kleine Fächer (so there are other people using this term). This features a list of rare subjects, such as:


Albanology
Applied nuclear physics¹
Crystallography
Mining science
Rhetorics
Sign language
Film studies
Glaciology
Forensics
Medical physics¹
Ethics of medicine¹
History of science


As for distinguishing these subjects from subfields of other fields, that site’s definition particularly requires that the subject in question considers itself an own field, having its own societies, journals, and particularly own curricula at university level.
So, your typical specialisation of a big subject does not count here because practitioners of such fields first start the big subject and then specialise as opposed to studying the rare subject as they enter university.

In colloquial German you would rather call these Orchideenfach, but I suppose practitioners of the subjects avoid this term due to its negative connotations.

Please mind that I am mainly describing the usage by relevant sources and quote their examples. I do not completely agree with this.



¹ For which it is unclear to me how they meet the definition given by the site itself, despite having one of the respective institutes next door to me and having specialised on another of the fields to some extent.