Declare reserves and transports section in the relevant
GT pack.
Nephilim
Each Dedicated Transport unit from your army must start the battle with one or more friendly models embarked within it. If this is not possible, that Dedicated Transport unit cannot be set up and is treated as having been destroyed.
Arks of Omen
In an Arks of Omen: Grand Tournament mission, each unit from your army with the Dedicated Transport Battlefield Role must start the battle with one or more friendly models embarked within it, or it cannot be set up and will count as having been destroyed during the first battle round.
Edit to add: Seriously, read the whole of each new
GT pack properly and regularly for two reasons, (1) to find the changes like these and (2) to actually (re)learn these basic concepts.
I "know" the rules but every few games I will actually read and confirm them as well. So some games I'll read my spells, some I'll read the terrain rules, some I'll read through the various steps for setting up a game. You'll constantly find things you've been doing slightly wrong and sometimes find something you were doing very wrong.