Ok, thanks again, things are getting a bit clearer! Maybe Turbo Diesel Freak can pitch in when he got a moment - thanks in advance! With all the hoses this system looks rather complex compared to the traditional one. Probably this arrangement is more suitable for the engines with widely varied cooling load, did I guess it right? Otherwise it is hard to justify extra complexity and space for much bigger two-section "coolant degasser tank".
More complex would be accurate, but the end result is also more efficient.
A "traditional" cooling system has quite a bit of room for aeration, air pockets, and therefore has generally required excess capacity and resulting extra weight.
Any air in circulation or trapped in pockets prevents the coolant from absorbing heat energy from engine metals as well as shedding heat energy via the radiator and heater core metals.Curious consequence of the presence of the big degasser tank with substantial amount of air in it, is noticeable pressure differential between the system and atmosphere present in the cold engine as revealed by the hissing sound which occurs on opening the main cup. That same pressure differential must of course be there in traditional system too, but due to the little amount of air under the radiator cap, it is not nearly as noticeable on cap removal.
Usually the pressurized coolant cap has no pressure at cold (not running) temperatures. The primary function of pressure within the system (when hot) is it increases the boiling point of the coolant, as vaporized water (boiling) has the same effect as outside air (disrupts heat energy transfer) within the system. The degasser tank gives a place where trapped air can rise to the top, and be replaced by pure coolant exiting the tank at the bottom.
I started this thread because I am trying to pinpoint a mysterious leak which leads to substantial loss of coolant. I have seen an older thread here on the same subject, but it didn't look conclusive.
Go to your auto parts store, and buy some UV dye for use in the coolant system, and buy or rent a cooling system pressure tester. Add the dye and run per directions.
When cool enough, remove all four glow plugs.
Replace the coolant cap with the cooling system pressure tester.
Pressurize the system to 14psi, and leave for an hour or so.
If pressure stays constant, then there is no coolant leaking, probably the cap has failed and allowing coolant to leave via vaporization.
If pressure has substantially dropped, then use the UV light and check everywhere around the engine, radiator, hoses, EGR cooler, Oil cooler, Thermostat, Water Pump, heater core(Inside too), reservoir tank, etc
If pressure has dropped and you can see no source of leakage outside, then pull the injector relay from the power distribution box, and crank the engine over (air pumped out through the glow plug holes will be extremely loud) Any head or head gasket leak into a cylinder will immediately show up via the coolant leaving through the glow plug hole.
If pressure has dropped, and no coolant comes out any glow plug holes, then either something was overlooked, or your leak is internal to the EGR cooler (exiting through the exhaust) OR exiting through the Oil Cooler (will show up via motor oil looking like a chocolate milkshake)Cheers,
A.
What I failed to understand was that despite being located within the same plastic body, the recovery tank was totally separated from the main reservoir, was open to the atmosphere and was connected to the main one with the thin hose going from its bottom to the side of the main cap. Thus it appears to be an exact equivalent of the expansion tank in "traditional" system. Phew! 
