Dynamic spill filtered based on other spill

Power Hely 

There is a catch.  The products associated with each account form a list, and so the table you are trying to create is a nested array.  Because of limitations build into Excel to retain backward compatibility, this is not properly supported.  The workaround is to use REDUCE and built the array step by step using VSTACK.

=LET(
    account,  UNIQUE(FILTER(MasterData[Account Name], MasterData[Rep] = SelectRep)),
    products, REDUCE("", account,
        LAMBDA(list, ac,
            VSTACK(list, TRANSPOSE(FILTER(MasterData[Product], MasterData[Account Name] = ac)))
        )
    ),
    HSTACK(account, IFNA(DROP(products, 1), ""))
)

It is possible to make the worksheet formula more readable by using a further Lambda function to return the lists of products using

= LET(
    account,  UNIQUE(FILTER(MasterData[Account Name], MasterData[Rep] = SelectRep)),
    products, MultipleFilterλ(MasterData[Product], MasterData[Account Name], account),
    HSTACK(account, IFNA(DROP(products, 1), ""))
 )

but the simplification is achieved at the expense of having to define a further Lambda function MultipleFilterλ

= LAMBDA(products, accounts, selectedACs,
    REDUCE("", selectedACs, 
      LAMBDA(list, ac, VSTACK(list, TRANSPOSE(FILTER(products, accounts = ac)))
    )
  )