'Set up within...' - Conga-line?

[Xalemich]

So, If an effect states that I must place a unit within a certain range of another unit, do I have to place all models within that area, or do they just need to start their deployment within the given zone?

Concrete example is the Vanguard Wing, where you can quickly run out of sensible deployment ground if you play a lot of Liberators in a single unit. - Do all of them have to stay within 5" of the Prosecutors?

[AverageBoss]

Within means partially within. As long as one model from the unit has its pinky toe in range, it is "within". Similar to when you shoot at an enemy unit. You don't need to have every single enemy model completely within range of your attack to shoot them, just need to be able to reach a single dude.

If the ability in questions says "fully within", such as deploying reserve units via gravesites with the new undead stuff, then every model must be placed 100% within range.