I think it's the new "Hybrid" terminology introduced in N23 (rulebook).At work so can’t check which, but one of the scenarios had a variable gang size 3+4. Clearly a typo for D3+4 but we all enjoy a bit of pedantry.
HYBRID SELECTION (X+Y)
When a scenario instructs players to use Hybrid Selection (X+Y) to choose their starting crew, the following method is used. When Hybrid Selection (X+Y) is in use, there will always be two numbers shown in the brackets:
• The player chooses a number of models equal to the first number (X) to be part of their starting crew and sets their Fighter and Vehicle cards to one side.
• If any models have multiple cards, the player chooses which equipment load-out the model will use for the battle.
• Models that are unable to take part in the battle may not be selected.
• The player then makes a deck of any remaining Fighter and Vehicle cards.
• If any models have multiple cards, randomly determine which card is added to the deck.
• Models that are unable to take part in the battle may not be included in the deck.
• The deck is then shuffled and a number of cards equal to the second number (Y) shown in brackets are dealt out and added to their starting crew.
Better luck next time