Difference between LIB_DEPENDS and RUN_DEPENDS

Hello folks,

Maybe an unusual request but could somebody elaborate on the difference between LIB_DEPENDS and RUN_DEPENDS?
I read the corresponding handbook section thrice and it's not clear to me when to use which - I must be missing something obvious.

RUN_DEPENDS appears to be for runtime dependencies. But when do I want to use LIB_DEPENDS?
I'm working on a port of a C++ library which depends on on some shared libraries (provided by existing ports) but it's not clear to me whether those should be listed in RUN_DEPENDS or LIB_DEPENDS.

Best regards,
~ joel