So, please where in the packaging guidelines can this be found?

https://en.opensuse.org/openSUSE:Package_group_guidelines """System/Libraries is intended for packages providing the part of libraries necessary to run applications. Packages in this group should already be installed automatically because of a dependency. Therefore, this group ought to be never used for the main Group: line of a .spec file (since the libraries ought to be in a subpackage, see SLPP). Neither users nor developers should need to search for packages in this group."""

Which exactly proves my interpretation: - there is no main package, so this is not violated - the library is packaged according to the SLPP and has the right group set

"Ought to be never used" is a weak statement which is justified by the fact the library is not in the main package, but other stuff is. The Group for the main package is guided by its contents.

What is the right main group if the spec builds only a library subpackage? What if it also builds a devel subpackage?

The guidelines are meant for build packages only, the whole reasoning is about packages which are installed.

If there were any policy about SRPMs, this should be clearly stated in the guidelines. But whats the correct Group for a SRPM creating library, documentation, devel and example subpackages?

What is the right main group if the spec builds only a library subpackage?

Easy: the topic/field in which the library operates. Same document,

"""If a library can be attributed to a specific RPM topic, use it. For example, the ntl library focuses on specific aspects of number theory, and so [is] under Productivity/Scientific/Math, while a general-purpose library like libHX is in the fallback group Development/Libraries/C and C++. For packages that fall inbetween two groups, pick the one that is less abstract, e.g. pick Math over Libs/C++."""

Care to cite the whole section ...?

It is about build packages - "This section [Development] has been created to help software developers. Normal users should never need to select packages from this group manually." ... "The following groups are intended for packages that allow developing with a library."

If a SRPM is meant for sw developers, all SRPMs should go into a Development subgroup. As this is clearly not intended, it does not apply to SRPMs. SRPMs are not for "developing with a library".

"The -devel subpackage is generally put into the group Development/Libraries/(Subgroup), depending on language. The subpackage providing the shared libraries located are generally marked as System/Libraries."

If you want the guidelines to be interpreted in a specific way, make the wording clear, specific and unambigous - after putting it up to discussion on the mailing list.

SRPMS are developy stuff, but if we made the Group a Devel/Something, then it would be wrong on the main BRPM too, if and when such is created. So the main Group: tag has to be the "topic" (Scientific/xyz, Graphics/abc, etc) out of necessity. This aspect gets reviewd in factory for over a year already.

If you feel the wiki is not adequately expressing already-applied practices, feel free to augment it, and if you need more detail, you can always ask about it.