Sure.<br /><br />Scenario #1:<br /><br />MinPlatformPkg/PlatformInit/PlatformInitPei/PlatformInitPreMem.inf and MinPlatormPkg/PlatformInit/PlatformInitPei/PlatformInitPostMem.inf both link against an instance of BoardInitLib.<br /><br />Many boards link against a single BoardInitLib instance. See example - https://github.com/tianocore/edk2-platforms/blob/cd4e6b716c7d1bcde94035e7dce14b53a553e103/Platform/Intel/KabylakeOpenBoardPkg/KabylakeRvp3/OpenBoardPkg.dsc#L203<br /><br />That BoardInitLib instance may link against SmmAccessLib. PlatformInitPreMem may wish to library class override the SmmAccessLib to the NULL instance while keeping it to non-NULL instance in PlatformInitPostMem.<br /><br />Scenario #2:<br /><br />A PEIM is built that checks whether the boot mode is S3. If so, it calls PeiInstallSmmAccessPpi(). A particular platform does not support S3, therefore, it links BaseSmmAccessLibNull as its library instance for SmmAccessLib.