Unless and until the DM rules otherwise, the rules in the books apply.
If there are no rules or the rules are unclear then rulings will generally be made in favour of PCs being heroic. Anything cheesy will be ruled against, however.
Except as noted in the following pages, all rules in the Player's Handbook (PHB), the Elemental Evil Players Companion (EEPC), and the Sword Coast Adventurer's Guide¹ (SCAG) are available. This includes feats, multiclassing, variant humans, and custom backgrounds. Some rules from the Dungeon Master's Guide (DMG) are in play, like scroll mishaps.
If you want to take backgrounds from the various Adventurer's League character option backgrounds then ask me about it. Some will apply to this game.
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¹ Since this game is set in the Unapproachable East, a thousand miles away from the Sword Coast, some of the rules in this book won't apply.