XCOM was to repel the extraterrestrial threat, beat the Ethereals in the Ethereal Subjugation War. The XSDF is meant to regulate the new technology, defend Earth and its interests beyond the planet, and so on. They're a supranational body that guards humanity, basically. There's an entry on the lore page that describes them if you missed it.
Ah okay, thank you for the explanation I'm sorry that I'm asking all these questions just curious, and yeah I just flew over the lore picking out small bits and such I will get to reading the full lore soon ^^