Carlo was in charge of shipping containers at the docks, I would not say he controlled the docks, otherwise he would be bringing in more money and Tony would not have made that out burst and disrespect in the car. He was given the construction racket later on, and seen as a consultant on a movie, counting recite. We dont see much of what he does really, but some of his actions and references indicate an accountant, possibly a shylock, and hijacking. He is one of the few people in the show where we do not know if he is a widower or has a mistress. Mob work in the show Fat Dom was heavy work, him taking product from shipping containers before they arrived at their final destination, accounting, relaying envelopes, which brings up another motive for turning as he gave Bobbies envelope to Tony who did not show support for Bobby earning even though he was in the hospital. Then you also have his cousin Burt being killed, instead of a sitdown, with Sil acting ahead without notifying Tony first, and Tony just nodding his head. Respect and protocols where not being kept during Tony's reign, and besides the Fat Dom incident which was in the heat of the moment, Carlos character was a stickler for the rules.