A Bounded Field was a topographic type technique that involved the user knitting boundary lines with universal law energy around a particular space to separate its inside from the outside, cutting off and isolating that space from the outside world and then imposing an effect on the inside.
It was the building block for an ability that any higher existence with a 'Territory' possessed: Domain Declaration.
In the event that they were stuck in an unfavourable environment for battle, they could just declare the surrounding area as their Domain to tip the scales in their favour.
Putting it bluntly, Domain Declaration was just a higher level of Bounded Field creation, where space itself acted as the boundary line.
Of course, there were other things involved like authorities and law energies and so on, but the base was the same.