Star Fleet Battles: Captain's Module P2 – X-Ships

From the first page:

This is the second of a series of modules containing Playtest materials from future Star Fleet Universe products.

This module is entirely composed of X-ship material, and is (in effect) a prototype for Module X1. While it is incomplete and unfinished, it is the most complete set of X-rules ever published anywhere (although these rules and ships are still subject to change in Module X1, based on your comments).