What is the role of UDC 00/DT ?

What is the role of UDC 00/DT?

My understanding is that DT or Transaction Document is only used to generate the Next Number. UDC 00/DT otherwise is a dormant feature in JDE E1. Is this the correct statement?

I added a new record called 9R in UDC 00/DI, but I didn't add 9R in UDC 00/DT. Standard Invoice Entry application did allow me to enter an invoice. Because, the Next Number constants was set to method 0.

Next, I changed the Next Number Constants from method 0 to method 2 viz. Next Number by Document Company and Fiscal Year. Now, I tried to set up the separate Next Number series for 9R. This time, Next Number application displays an error, as I didn't update 9R in the UDC 00/DT.