The Votaan codex has a special requisition available for Crusade missions:
IMPERIAL MERCENARIES: Purchase this Requisition at the start of any battle if your Crusade armies contains both one VOTANN Combat Patrol Detachment and one or more of the following units: INQUISITOR CHARACTER, ASTRA CARTOGRAPHICA CHARACTER or ASTRA MILITARUM CHARACTER. Until the end of the battle, all VOTANN units in your Crusade army gain the AGENT OF THE IMPERIUM keyword.
Then referencing the AGENT OF THE IMPERIUM rule from the Astra Cartographica section of War Zone Ocarius: Critical Mass:
If your army is Battle-forged, you can include one AGENT OF THE IMPERIUM unit in each IMPERIUM (excluding FALLEN units) Patrol, Battalion and Bridage Detachment in your army without those units taking up slots in those Detachments. The inclusion of an AGENT OF THE IMPERIUM unit does not pervent other units from their Detachment benefiting from Detachment abilities (e.g. Chapter Tactics, Defenders of Humanity etc.), and it does not prevent other units from your army benefiting from abilities that require every model in your army to have that ability (e.g. Combat Doctrines). An AGENT OF THE IMPERIUM unit including in a Patrol, Batallion or Bridage Detachment in this manner is ignored for any rules that state all units from that Detachment must have at least on Faction keyword in common (e.g. in a matched play game), and when determing your Army Faction
I am struggling to understand the intention of this requisition. I suppose your crusade roster could have a mostly Astra Militarum army but also a handful of Votann units (there is a separate rule that explicitly allows this). You could then use the requisition to take one Votann unit for each of your detachments without breaking anything. But you have to pay a requision point. Every single battle where you want to do this. And if you only have one Militarum detachment then you only get to bring one Votann unit. And if you don't pay then your roster is cluttered up with multiple Votann units (enough to fill a Combat Patrol) that you cannot use. Or have I misunderstood?
The situation gets more absurd when you consider the Inquisitor and Astra Cartographica options. Do they really think that someone is going to take a full Astra Cartographica army with a handful of Votann allies? Or is the idea that the Rogue Trader is using her influence to force my Deathwatch army to take some Squats along? But they can't both come at once unless I have multiple detachments. And now my crusade roster has units from three disparate factions...
I just can't get my head around what they were going for. It could be just to give the option for flavor but it seems such a terrible deal that I still can't see anyone doing it. Am I missing something?