Except dataminers (and people who use a certain -engine- program) already confirmed long ago the buff's internal mechanism in the code showcased it's a binary flag; it guarantees the server picks you for CE's, unless there's at least 48 others who have the buff, or an equivalent 'Marching Orders' from fate-specific ones, then the game RNG's who gets in from the buff wielders alone. The thing is, Castrum isn't classified as a CE internally. Whether intentionally or as a bug, Marching Orders doesn't work on it. So a visual demonstration on how Marching Orders works on all non-Castrum CEs:
>Server checks all players who queued.
>Sorts queuers into two groups: those who have a marching buff and those who don't.
>Takes all marching buff players and adds them to 'successfully queued' array
>Checks to see if 'successfully queued' array is full.
>If no, randomly choose players from non-marching orders array till it reaches 48.
>if yes, it then checks to make sure the selected players isn't over 48.
>if they are, it randomly chooses who gets kicked out until it has 48.
Beyond that, Square is often extremely picky with their word choices for buffs. if it says 'Priority', a word that can be defined as 'the right to take precedence or to proceed before others.', it ain't hard to figure out that it's ignored on castrum when it doesn't let you in. If something isn't guaranteed, they usually word buff text to say something akin to 'increased chance'.
IIRC, Duel's aren't classified as CEs either, and Marching Orders gets nullified on those as well, so there's already precedent through visible observation in Bozja itself.
It scales to a point. It stops scaling downward roughly at about 20-24 players, but it does scale upward all the way to 48 players.
The thing is, its scaling isn't exactly a 1:1. Theoretically, a full group of 48 players doing proper rotations, essences + lost actions would completely annihilate the bosses even compared to a group of 24 doing all the same things, since the boss HP scales around a player doing average dps for their job, well below the max output of a job's rotation.
But in practicality, the rate that the bosses die between smaller groups and larger groups will be about the same since people will be missing essences, missing lost actions, not doing proper rotations, etc. The more players mostly equates to extra ressurection power to prevent wipes.