The Deanna Note was sent by Detective Ritz to AT&T. It requested that AT&T provide maps plotting out the cell towers that Adnan's phone had made calls on, in order to "aid & corroborate information provided to us by witnesses an discredit the suspect's alibi."
Ritz did exactly what he said he would do with the information AT&T provided: he used it to aid Jay's statements, so that the cell records could be used as "corroboration" for his testimony at trial, even though that testimony had been directly used in developing his statements.
Unfortunately for Ritz, either AT&T or the police department made a mistake in plotting out the cell towers. Tower L654, which is located just south of Westview Mall, was incorrectly located three miles to the south -- at a place very close to Cathy's apartment. As a result, Jay's statement adapted to reflect the incorrect maps, and he told the police what they wanted him to say: that at the time calls were made on L654, Jay was at Cathy's apartment.
For more information on how the mistaken placement of L654 is evidence of the police coaching Jay's testimony, please see Evidence that Jay’s Story was Coached to Fit the Cellphone Records," at the View From LL2.