This is a general rules question. However, my belief is that the answe is "no". Regardless, expect table variation.
How can organized play even work when "Is my basic character concept legal?" is something subject to table variation?
And if the player's only level 5-10 character is their Urchin Purple Dragon Knight (born to nothing but earned her way into the nobility as a teenager by saving a nobleman's daughter's life and being adopted in return) and the game being run that day is in that tier... what then?I suspect this will be something that gets admin attention, but until then, you do what you would normally do when joining a new game -- check with the DM first. If your character isn't going to work, pick a different character.
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Pauper
So... you'd throw my above-example player out of your game in that situation because they didn't follow a rule that's not actually written anywhere? And you recommend that people pick their option from a list a random member of AL staff posted to a third-party forum?After looking at it further, I would personally allow any of the following options at my table:
Noble (Knight)
Knight of the Order
Custom Background (Custom Knight, eg. Hedge Knight)
It appears that the intent of this class archetype is similar to that of the Battlerager and Bladesinger, in that only characters who meet a specific requirement (in this case a limited list of backgrounds) can become members. Since both the battlerager and bladesinger are restricted in AL, it stands to reason that anyone playing a Purple Dragon Knight or Banneret in AL and is worried about table variation should stick to one of the above three options (as that way, they will never run into issues).
So... you'd throw my above-example player out of your game in that situation because they didn't follow a rule that's not actually written anywhere? And you recommend that people pick their option from a list a random member of AL staff posted to a third-party forum?