Answer: 1. the case built must be new and 2. there must be less than 500 pieces.
The best way to mass-produce watch cases with industrial alloys (brass, steel, titanium or aluminium) is to gradually press-stamp raw blocks into shape. Operations have to be split into several steps because each stamping creates tension in the alloy, which then needs to be heated to release said tension. When they are too tense, alloy blocks can damage the moulds or they can themselves break.
Based on the intricacy of the shape that needs to be achieved, watch case designs require between one dozen and two dozen pairs of press moulds. All these moulds cost more than several thousands of dollars, so using that process for, say, 10 watch cases is really counterproductive. Merging the cost of tooling to the price of raw alloy would drive the cost of each one of those cases to the sky.
This is the reason why most case makers usually manage to break costs even beyond 500 cases.
As a consequence, if a brand announces a limited edition of less than 500 pieces (needless to say: with a brand new case design) there is a high probability that these cases were brought into shape through matter removal (such as CNC lathing or electro-erosion).
On the other hand, if a brand announces a limited edition of more than 500 pieces, they are in the grey area because press-stamp tools do not wear out before at least 5000 cases have been produced.