There are a variety of libraries that serve special collections. These are known as "Special Libraries"
This particular association focuses on the need of member that work in academic, corporate and governmental library professionals. I'm getting the sense reading through the website they make it clear that they are really focusing on Information professionals.
In today marketplace that is not just restricted to librarians but to those individuals that use information as a method of supporting the goals of the company that employ them.
For example, News Librarians use all of the skills of research and information gathering but those skills are using to support the newspaper, magazine or broadcast news industry.
There is also the Transportation division of that focuses on transportation issues. This will include air, land, water, rail mass transit and a host of other information research.
The SLA is a membership organization that provide job and career information, research and networking opportunities to the membership.