A legal expert was a generalized expert in understanding the law.
When first negotiating the Treaty of Armens with the Sheliak Corporate, the Federation sent three hundred and seventy two legal experts to ensure its completion. (TNG: "The Ensigns of Command")
After Miss Sarda approached Commander Benjamin Sisko about the contract she signed after being hired as a dabo girl at Quark's, she admitted that "I guess I should have read it more carefully. I'm not a legal expert, Commander." Sisko, too, admitted that "I'm not a legal expert either..." but went on to assure her that after he talked to Quark, no one else would be held to the provision concerning the exchange of sexual favors. (DS9: "Captive Pursuit")
When the Q (later known as) Quinn requested asylum aboard USS Voyager so that he could commit suicide, he sought out Tuvok to represent him in the hearing. Tuvok was confused by the request because he had no legal expertise, but Q(uinn) insisted, citing the fact that he needed "someone who understands Federation asylum practices," and also the fact that "Vulcans approve of suicide." Though Tuvok failed to completely see Q(uinn's) point, he accepted after it was pointed out to him that "I have the right to counsel, Mister Tuvok. Will you assist me?" (VOY: "Death Wish")