The word Shareplex instance is used frequently and interchangeably with port #, variable directory (vardir) and product directory (proddir). This solution tries to clear the doubts surrounding the exact terminology.
A Shareplex instance is an entity with its own queue structure and processes. These two variables uniquely identify an instance. The only thing that can be qualified to be closely tied to a Shareplex instance is the vardir. This is the reason the correct term that can be used interchangeably with a Shareplex instance is the vardir. The vardir is also pointed to by the environment variable SP_SYS_VARDIR.
The port # is sometimes used as a substitute for Shareplex instance. But in a true sense it is not fit to be used as a proxy for an instance. The reason being it is quite easy to change the port # of an existing instance (see SOL25822 for details). So a port number's association with a Shareplex instance can be changed with ease. This is not true for a variable directory.
The proddir cannot be used to uniquely identify a Shareplex instance because it is quite common for a proddir to be associated with several instances. The proddir only contains Shareplex binaries and does not contain the queue structures or other entities that spawn Shareplex processes, something that vardir does. Hence vardir is the only unique identifier for a Shareplex instance.