I think a linear battlefield is one in which there is a "front line", which was a given for prior wars but maybe not as much anymore. Future wars might see adversarial forces mixed among each other spatially more than in the past.

Static (relatively, at least) battle lines? Think of WWI: you knew where the danger was, and where it wasn't.

Not that I know anything in particular about this piece of military jargon, but that's my contextually-informed supposition.