You can transfer between PBX user extensions.
There are some system extensions on each side. These are unique to each system and in many cases only the extensions connected to a given PBX are the only extensions that can use that resource.
The example that was used was PARK. If you place a call into Park on one PBX, an extension connected to the other PBX is not able to retrieve the call. There are others such as system stored Speed Dial, and there are some workarounds that might be employed for some, but suffice it to say that there are some resources that cannot be shared between the two systems, but calling to user extensions can be done, transferring can be done, calling thru to use the other system’s trunks can be done and so on.