Consider this example

// translation C 
export module M :C;

// translation U
export module M:U;
import :C;

// translation unit T
export module M;
export import :U;  // #1

[module.unit] says

All module partitions of a module that are module interface units shall be directly or indirectly exported by the primary module interface unit ([module.import]). No diagnostic is required for a violation of these rules.

[module.import] p7 says

When a module-import-declaration imports a translation unit T, it also imports all translation units imported by exported module-import-declarations in T; such translation units are said to be exported by T. Additionally, when a module-import-declaration in a module unit of some module M imports another module unit U of M, it also imports all translation units imported by non-exported module-import-declarations in the module unit purview of U. These rules can in turn lead to the importation of yet more translation units.

It is clear that the U is exported by T since an exported module-import-declaration at #1 imports U in T, which is exactly said in the first emphasized part.

Since T and U are module units of M, the second emphasized part should apply to the module-import-declaration at #1, since the non-exported exported module-import-declaration import :C; appears in U, the module-import-declaration at #1 should also import C. So, C is arguably said that it is indirectly exported by T. All partition module interface units of M are exported by the primary module interface unit of M. There is no violation here.

However, If we compile the example through GCC, it reports an error that:

interface partition is not exported

Not sure whether it is a bug of GCC or [module.unit] falls behind the implementation. Anyway, I think directly or indirectly exported where these adverbs should have their English meanings. U is "directly exported" by T means that U appears in an exported module-import-declaration in T. U is "indirectly exported" by T means that U does not appear in an exported module-import-declaration in T but it is imported by the module-import-declaration.

Wait for the confirmation of the meaning of [module.unit]. If it is a bug in implementations, I will report that bug to the vendors of implementations.