The Resistance is a loose term for describing the various pockets of humanity not in the UCM. Some have eeked out an existence on planets under Scourge control. There are many of these pockets, some are now at an almost tribal level of society where the strongest survive and the weak ar left to die. They all share one common purpose, to destroy their Scourge occupiers. However they don't always get along and many dislike the idea of the UCM taking control of them. Others are the Vega Scrapfleet, survivors of the Battle of Vega that didn't join up with the others that became the PHR and didn't escape to the frontier worlds to join the UCM. Theer is also the Kalium Cabal, humans who escaped to the Frontier Worlds but had no interest in joining the UCM. The Kalium cabal have an impresive fleet and their assistance to retake Earth is crucial to the Reconquest.