The notion of an "absolute form of war" is very clearly derived from the new style of warfare introduced by the French Revolution and brought to a high level of performance by Napoleon Bonaparte.
Although it is presented as a philosophical concept, and thus probably not capable of perfect attainment in the real world, it was nonetheless modeled very closely on Napoleon's most masterful campaigns and was intended to be pursued or emulated by commanders "when possible."
It contrasts to a weaker, less competent—even pointless—form Clausewitz called "war of observation," based on the carefully circumscribed use of force in the century or so prior to the French Revolution.
In Book I, drafted later, the term absolute war does not appear; for practical purposes, war in the real world is described using a spectrum bounded by the limited aim of wearing down the enemy's will to carry on the military struggle versus the most ambitious form available in reality, the aim of rendering one's opponent militarily helpless.
Therefore, war in its most logical form would involve each state continually reciprocating each other's use of force (plus some) to maintain a superiority, until both were using violence to its utmost extent.
Once a state has learned the enemy's strength of resistance it can review its own means and adjust them upwards accordingly in an effort to gain the advantage.
Thus "absolute war" reflects an earlier conception advocating the more extreme reaches of the forms that Napoleonic warfare had actually achieved.
Clausewitz never seems to have resolved whether absolute war was supposed to reflect an achievable form or was instead an ideal to aim at, but just out of reach.