Ship locking is a system that restricts to which maps a certain ship may be sortied. When a ship is sortied to an event map for the first time, it will have a tag applied to it. The Tag System prevents any fleet with ships that have a tag different from the one that the map accepts from sortieing to said map. In Easy and Casual difficulties, tags are still applied, but are not checked, meaning you may still sortie fleets with ships with different tags, however, the map's tags will still be applied to ships without any. Ships that are already tagged cannot lose it through any means.
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Furthermore, a map might accept any or multiple tags, and as well not issue any tags.
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===Difficulty===
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Exclusive to event maps, a difficulty can be selected. The difficulties are referred to as "classes". Classes go from D (丁) to A (甲), with the western standard for difficulty being Casual (丁), Easy (丙), Normal (乙) and Hard (甲).
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Difficulty changes the enemy compositions, routing rules, and map rewards accordingly, as well as ship drops tables with both their rates and availability being affected.