When edX launched its first few courses, there was no Studio with a user interface for setting up courses. Instead, courses were developed using edX's "open learning XML" or OLX. When Studio was released, not every capability that was available through OLX was duplicated into the UI. Many of the courses that MIT launched early on are still being re-run today, and they can and do contain unique or rarely used features that are only available by editing the underlying OLX.
After promoting the use of Studio pretty much exclusively to newer partner course teams, in mid FY2015 OLX was again thought to have value for a wider community. An MVP of the OLX guide was written very quickly late in FY2015, to expand adoption. However, interest then waned, and the MVP was never finished. Numerous gaps remain, and there has been no emphasis on keeping what was published up to date.
The "OLX guide" is something of a misnomer, as JSON files are also described.
Technically-minded course teams are the main audience.
The RST source files that define the structure of the OLX guide are in the GitHub edx-documentation repo, in the { language }/OLX/source directory. You can use Sublime or another text editor to edit the RST files, and render and publish HTML and PDF with ReadTheDocs (the project is "edX Open Learning XML"). See Documentation Tools.
In an effort to reduce duplication, the files that describe the use of OLX in the advanced editor for core CAPA problems, are also used in the two versions of the Buiding and Running an (Open) edX Course guide. The files in the { language }/OLX/source/problem-xml directory that describe CAPA problems take advantage of the RST include directive.
For example, if you open the checkbox.rst file, you see a single line directing you to another file:
.. include:: ../../../shared/exercises_tools/checkbox.rst
course_authors/source/exercises_tools/checkbox.rst file and the open_edx_course_authors/source/exercises_tools/checkbox.rst file.
Use of the include directive is an important tool for reducing errors and translation costs, but it does mean that a single file can be serving multiple audiences, and should therefore be approached and edited with healthy respect for possible consequences.
(Course structure information was added to the Researcher's guide after it was written for the OLX guide, and file sharing with the include directive is not used for information that overlaps between the guides.)