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Initially, we announced this change and some of the reasoning in early December: Lost Ark December Team Update: Sorceress - News | Lost Ark - Free to Play MMO Action RPG
When it comes to choosing class order looking ahead, we touched on this in our rodamap – "our teams work with Smilegate RPG to evaluate a variety of factors when looking at which Advanced Class to release at a specific time, such as planned balance updates/reworks, relevance of the abilities and playstyle to other content, community feedback, and more.” This is also true for the Summoner moving ahead. I understand players are looking for a more specific answer, but unfortunately I can’t reveal additional details at this time. I will provide more context as soon as I am able.