Prior to JIRA Client 3.1, there were different editions of JIRA Client - Pro, Lite and Site, with each edition designated to be used with a specific type of license.
Starting with JIRA Client 3.1, there's only one download, regardless of what kind of license you have.
|Version:||3.5.1 (Release Notes)|
|Release Date:||November 29, 2013|
|JIRA versions supported:||5.0—6.2+|
JIRA Client requires a license to run. If you're evaluating JIRA Client, please use one of the following methods to install an evaluation license: