When a TPH is properly registered in the Windows registry, and the client is started, GroupWise loads the TPH DLLs listed in the registry. For each TPH it loads, it calls the Compatibility() and Entry() methods. Note that all TPH DLLs are loaded before C3POs are loaded.
The Compatibility() method lets a TPH specify the GroupWise version that can load it and ensures compatibility between GroupWise and TPH DLLs. The TPH can verify major and minor GroupWise client versions, then return to GroupWise a value indicating whether to unload the TPH or not.
The Entry() method is where your TPH sets up its necessary data structures, memory allocations, performs initializations, etc. It also tells you the language under which GroupWise is running. Like the Compatibility() method above, if the TPH does not support this language, the TPH can then return to GroupWise a value indicating it should unload the TPH.