Thanks for the responses!
One of each really.
Count: I understand what you are saying, that an objective cannot be placed 'inside' a line, but behind it. However, the view is that because the Defence Line is brought as a fortification, it is not just a barricade or wall, it is also granted the bonus to cover save when going to ground. While a unit can be inside a fortress or bastion to gain its benefit, for the ADL being 'inside' is being behind.
Could also be part of configuration I suppose. If I placed a unit of Wracks behind an ADL in a three sided semi-circle, or even fully enclosed as shown on Pg. 114 BRB - I should be allowed to do so because it is 'behind' the ADL and not 'in', am I correct in understanding that to be what you are saying?