In the guidelines, this is what it says:

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 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++.

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.

So it is less than obvious what the first paragraph is referring to, the whole meta-package Group or the devel package. I interpreted the second paragraph as being only applicable for general-purpose libraries, i.e. when Productivity/Scientific/Math is not appropriate. Anyway...