Accordign to FMCSA, ELDT is required to:
1) Obtain Class A or Class B CDL for the frist time;
2) Upgrade existing Class B CDL to Class A CDL; or
3) Obtain School bus (S), Passenger (P), HAZMAT (H) for the first time

This means adding endorsements for Tanker (N) or Double/Triples (T) is just the knowledge test.  IIRC when I took tanker it was 9 questions and 3 where about outage.