AddHits and SubHits
NOTE: This technique is not needed for simple games, so it is not mandatory for jr-devs.
AddHits and SubHits can be chosen in the Flag column of the Achievement Editor.
The AddHits/SubHits conditions do not have to be true for the achievement to trigger. They modify the hit count of another condition, which does have to be true for the achievement to trigger. The sequence of conditions is referred to as a AddHits chain (even if it contains SubHits).
When a condition with the AddHits/SubHits flag is true, the hit count for that condition is incremented. A hit target may be placed on the condition to keep it from tallying too many times.
The total hit count of the final condition in a chain is incremented by the hit counts of any AddHits conditions in the chain, and decremented by any SubHits conditions in the chain. If the total hit count meets or exceeds the target hit count of the final condition, it's considered true.
If the final condition of the chain does not have a hit target, the AddHits/SubHits conditions have no effect on the achievement. The final condition will be true if, and only if, it is true by itself.
NOTE: The hit target on the final condition of the chain serves as both the overall target hit count and the maximum hit count for that condition. As a result, it is impossible to use SubHits without AddHits, as any hits subtracted by the SubHits could not be recovered without a separate AddHits. Similarly, if you need to put a condition-specific limit on the final condition of the chain, you should pull it out as an AddHits condition with the appropriate limit and use a dummy final condition with the overall hit target (i.e. 0=1 (10)). The dummy condition would never accumulate any hits of its own, and only serve to tally the other values in the chain.