The original manuscript for HARP SF was a big book. ICE wanted more detail on the setting. The book got bigger. The entire book went into a public beta round of playtesting. Mistakes and errors were purged, more detail was added where necessary. When all input from playtesters had been incorporated, the final version *4i* went back to ICE.
The complete work was too big for single volume publication, at least for distribution purposes, so ICE divided the book into two parts. The first part is HARP SciFi, which covers the universe, characters generation, equipment/technology, personal combat, adventuring, psionics, and SysOp [GM] advice. The second part (which is about one-third of the manuscript) is HARP SF Xtreme, which covers vehicles [from groundcars to starships], vehicle combat [for space battles], cyberware [neural implants, turning characters into cyborgs, etc.], and cybertech [creating AIs and robotic characters, turning biological characters into personality downloads].
You want both volumes for a comprehensive sf game.
Best wishes,
Nicholas