Creating a N:1 Relationship

Hello,

We're customizing our system and want to make the following changes:

We have an entity called Truck. We want the truck to have line items, but we don't want these line items accessible by any other truck. We add a few line items OK.

Now, we go and add another truck and then add line items and it shows Add Existing, and shows the existing line items we made before. With a N:1 relationship, doesn't that mean many items (line items) may only be linked to one item (truck)? a 1:N relationship would be one line item can be on many trucks. So confused!