Could you have also made the LetterActiveR with an SQL statement based on the CustomerT with a WHERE condition about IsActive being True (If you didn t want to make a separate Query)?

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Reply from Alex Hedley:

Yes you could, but it's sometimes neater to have a saved Query, and easier to see what you have set, if you have to go into the RecordSource and look into the SQL it gets a little bit fiddly.
A Query doesn't take up much memory and although your db can grow in size with the number of objects, Rich does teach in later classes how to use SQL strings or VBA to slim down the number of objects in your Navigation Pane.

LetterActiveR is based on CustomersActiveQ which is CustomerT with a Criteria of IsActive=True