figuring out the output language is where a lot of the domain knowledge comes to play. although at the end the output language is just whatever you need it to be for your task. it may have a very small vocab size depending on what you are trying to "translate" to. for us it is a sequence of words and numbers that express the various types of calendric intervals that corresponded to the input emails. teaching human labelers this sort of task is challenging if you want to get high-quality labels. we had a team of in-house labelers labeling data (the human-in-the-loop) and we closely interacted with them and monitored the quality of their data and gave them feedback through various methods. we also perpetually improved guidelines for them. it was a pretty intense iterative process because we wanted their labels to be as accurate as inherent ambiguity in human language allows. so a lot of work went into that.