Incompatibility of Protocols. Give example

Shirshendu De

During my study I read a para saying ''Application layer protocols are used by both the source and destination devices during a communication session. For the communications to be successful the application layer protocols implemented on the source and destination host must be compatible''. What is the term compatibility or incompatibility would refer to in this context? Any example

kaiju

TACAS and TACAS+ are the first to come to mind for me.