One does not ordinarily think of librarianship as a profession with many specializations. In some fields such as medicine, law, and information technology there are clearly jobs which require a particular set of skills. An application developer does something different than a network administrator and a trauma surgeon is clearly distinct from an oncologist. Within the library world, the differences between job roles are seemingly less vast. Working in reference is not the same as working in serials or circulation yet one can learn those roles fairly quickly. Indeed many libraries combine those jobs in various ways depending on the need of the organization.
However there are exceptions. Cataloging stands out as one. There are a great deal of rules one must learn in order to effectively describe library materials. Additionally one must master the MARC format as well as tools such as MARC Edit and OCLC Connexion. Furthermore the nature of the work is different than what most librarians do. It requires a highly detail oriented focus instead of the service one which most library tasks call for. So library cataloging is one area many librarians steer clear of unless they have a strong interest. Systems librarianship is the same way as well to a certain extent. In order to be a successful systems librarian, one must acquire technical skills. These can include everything from understanding how the ILS works to programming depending on the institution.
Yet the gaps between these branches are fairly narrow. Many systems librarians fell into the position by accident. There is even a book devoted to the subject. And some libraries combine job roles in all sorts of ways. So over time what were separate departments have drifted together. Titles such as Circulation and Systems are giving way to Access Services, Discovery Services, and Metadata librarian thus emphasizing the essential aspects of what it takes to operate a library. Plus we all get the same degrees. It's not always called Library Science, but the essence of what we study in graduate school is often the same.
So when considering if one is in a specialized field or job role the key distinguishing factor is education. If one needs extra education in order to do a job, then it is a specialized one. Thus the various branches of medicine are specialties because in many cases one either needs to know a particular body of knowledge, attend a certain school, or continue one's education beyond getting the degree itself. The same is true to an extent in IT. Many organizations (rightly or wrongly) require certifications in order to have certain jobs. Even without the certification different aspects of the field require learning new skills. For example a Systems Administrator may need to acquire in-depth knowledge of networking in order to become a Network Manager.
That is not to say all librarians are interchangeable. We're specialists in the way lawyers are. We all have the same education and anyone can take any job. However the more one does a particular job, the better one becomes at it. So an experienced reference librarian will fit most easily into a similar sort of role. Of course one can branch out and take on different responsibilities, but it is always easier to stay within one's area of expertise.
Ultimately there is a lot of politics in all of this. Defining oneself as a specialist is the first step towards arguing for better pay or more resources. One can start a professional association to argue for why their specialization needs to exist, be recognized, and why it should be funded. So whenever one defines themselves as being a specialist in something it helps to reach for the nearest grain of salt.
I'm not saying these issues aren't important - they are. In an era of declining or stagnant budgets libraries need to distinguish ourselves in order to continue selling the value of our profession. There may be few true specializations within librarianship, but the field itself is distinct within the broader information science world into which we are often lumped. Librarians provide access to information, tools to use said information, and assistance in making sense of it all. The services and spaces we provide can't be easily replicated elsewhere despite what broader cultural narratives may say. So in a sense we are all part of a specialized information science discipline, one rooted in service and unfettered access to knowledge.
