$CurrentLink is the LinkObject representing the MathLink connection for an external program currently being installed or being called.
$CurrentLink is temporarily set by Install and by ExternalCall.
You can use $CurrentLink to distinguish between several instances of an external program running at the same time.
$CurrentLink can be included in :Pattern: and :Arguments: MathLink template specifications, and will be evaluated at the time when Install is called.
See The Mathematica Book: Section 2.12.9.
See also: $ParentLink, Links, $Input.