IH bros please help my smooth brain understand how the Legion works.

So at the highest level we have Clans, which are approximately equal to Chapters but are independent and self-sufficient with their own forges, fleets, recruits etc. Essentially the Legion is actually ~50 micro-Legions all drawn from the same planet and led by the same Primarch.

However for actual fighting, they will create an Order to fight a specific campaign or battle, given their own ships/troops and expected to fight as an independent organisation. These are often, but not always, made up of Legionaries from a single Clan. They are equipped with everything they need for that campaign and nothing more; failure and/or asking for help is shameful to the point of ritual bionic suicide via Immortals.

My question is below that level; are Orders permanent or are they ad-hoc arrangements of whichever Companies are to hand? Iron Hands still use specialist Companies but they come together to make a generalist Order? I don't really understand what makes an Order different to a standard Battalion.