I don't fully understand Detachments

Apologies if this is obvious, I'm new to wargaming.

My understanding of Detachments is: when building my fire team, if I utilize units specifically mentioned in each of the two detachments, and do not use any units outside of the ones listed, I can use the Fire Team Upgrades.

The Special Orders are not exclusive to the Detachment, but they're listed in the Detachments PDF because those are the only ones I can use with that specific team.

Is this correct?