Correct me if I'm wrong, but this means that if you leave 200pts in reserve, and have the units available, you can decide which unit, be it an assault, tactical or HS unit, to bring on, so long as the final points total you have on the table does not exceed the game limit? That's actually pretty cool.
I think the 200 points is for summoning. In that you cannot summon past your starting army, so instead of building a 2k army + Summoning, you fielding a 1500 army with (500) as summoned points.
For instance a "summoned" unit for the Imperium could be LotD... or Kaldor Draigo?
That said if Summoning works across the board and it's effectively Deep Strike, then I might be worried, but if it is curtailed a lot so that it's more a Chaos kind of thing I would be very happy.
If you put a unit of terminators in reserve/deep strike/out flank then it will still be terminators.