There is no one system or set of procedures by which agencies investigate or adjudicate claims.  The Administrative Procedures Act (APA) contains some very basic guidelines regarding adjudications, but the format and protocol for such hearings varies from agency to agency.  The terminology used to refer to decisions in adjudications may be referred to as decisions or opinions, but they may also be called orders or releases.  Bluebook Table T1.2 includes a non-exclusive list of administrative agency and executive materials that can be helpful in determining what an agency calls its decisions and where they may be published. 

West's Federal Administrative Practice is a treatise that provides guidance regarding handling matters before Federal departments and administrative agencies.   It provides a nice overview of an agency's structure and their adjudication process.

There is no one publication for administrative decisions and they may be available in one or more several electronic sources. 

In addition to regulations and adjudications, agencies have a wide variety of other guidance documents that may be called subregulatory guidance, nonlegislative rules, manuals, handbooks, guides, advisory notices, letters from the head of the agency, etc.  In general, this category of information is often referred to simply as administrative guidance. 

If an agency has published such guidance, the agency's website is the best place to locate it.   The University of Virginia maintains an excellent website with links to agency guidance and other documentation.