Mimeo, Technical report, Manuscript: what kind of creatures are they? (I consider this question to be an extension and complement of this one).  

My field is Economics. It is not uncommon for a published research paper to reference a (third-party) "technical report", or a "mimeo" or a "manuscript", usually accompanied by the name of a University only (apart of course from author and a year as date). I know the meaning of these words, and that they represent something different from a "working paper" - the latter is almost always part of an instituted series of publications by a research or academic institution. 

My question asks help in clarifying how such documents come about as part of the activity of a professional academic, and why they are "left" at that "status" and not, say, "advance" to at least "working paper" status. After all, they are referenced because they contain research results.  

I can imagine some situations: for example a "Technical Report" could be the very detailed presentation of a large-scale econometric or simulation study, that was then condensed to a working paper and then maybe to a published one. Or some very specialized theoretical/mathematical result which has value but it is too narrow to even be published as a "note" (and perhaps belongs to some future monograph or textbook). Or even, these are works that have been submitted and rejected, but their author thinks they nevertheless have value, and it so happens that he stops trying to publish them and decides to keep them public in his university and/or personal website, as proof that he is fulfilling his obligation to do research.

Am I off the mark? How publicly accessible and referenced scientific works (even from recent years) come to settle in a "manuscript/mimeo/Technical Report" status?
Most often I find that these are works in progress, unfinished or otherwise unsuitable for regular academic publication. I've written technical reports as an undergrad as part of an internal project for a non-research entity that simply described the progress we made on a project. There was little context for publication outside the organization itself, and not enough novelty to try to pull some context together. However, these documents were referred to internally to justify further work in the area due to the feasibility that we showed for the project. Sometimes, when they're not too proprietary, some organizations will put these kinds of reports up on their website to show the kinds of things they are working on. Maybe several will be combined at a later point as the foundational parts of a proposal or a "real" paper. 

I haven't seen "mimeo" used ever in my life except in the context of the output of a Mimeograph machine which was an early way to duplicate documents. It's possible that mimeos of technical reports in the scientific report context used to get passed around the big national and industrial labs so that people could find out what was going on internally well before there'd be an internal website or wiki to browse for like-minded researchers. Otherwise, that's not really a common term that I encounter.