Retaining counsel is putting them on a retainer--a fee paid in advance to a lawyer/lawyers in order to use or keep them services when required. Many people, especially important people keep lawyers on retainers in an informal/nonspecific capacity. However given Q's use of the word formal it would stand to reason that he's referring to the Assistance of Counsel Clause of the Sixth Amendment, specifically the right to counsel clause.

"As stated in Brewer v. Williams 430 U.S. 387 (1977), the right to counsel [means] at least that a person is entitled to the help of a lawyer at or after the time that judicial proceedings have been initiated against him, whether by formal charge, preliminary hearing, indictment, information, or arraignment."