One example of things to be clarified would be manner of speaking and the actions that are acceptable in the game - like swear words, mature behaviour or even trigger situations. It is also possible to establish the boundaries of their roleplay in more simple matters for example, stating that the session is committed to completing a certain questline, or insisting that the game should be focused on non-combat issues. By being firm and clear about boundaries makes sure that everyone involved during the game have the most enjoyable time possible, without damaging each other's feelings.
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