Z39.50 is a standard that allows different types of operating systems containing databases to share information through a standard. It is not an interface, it is what establishes the rules that allows for this to happen. 

The standard is immense, and is established as a NISO standard by the Z39.50 Implementers Group (ZIG). The membership to the committee is very open, and ProQuest participates in monitoring and voting on important ZIG issues. 

The complexity of the standard means that each company or institution has differences in the way they instituted the standard. 

To learn more about Z39.50 go here: http://lcweb.loc.gov/z3950/agency/

EndNote® by ISI Research is one such interface to the Z39.50 standard.  Below is a sample configuration file and other information required to use EndNote® with ProQuest.

z39.50 database name and URL