The evidence by retired RUC superintendent David McConville, previously given in closed session, was read into the public record at the inquiry today.
The tribunal is examining allegations of garda collusion in the IRA ambush in which RUC chief superintendent Harry Breen and superintendent Bob Buchanan died.
Retired sergeant Mr Owen Corrigan denies allegations of collusion.
The Special Branch report, known as an SB50, was graded using a letter and a number, the tribunal heard, with the highest grading being A1.
Objections from the PSNI on the grounds of national security meant the actual grading was not placed on the public record, but the tribunal heard evidence about the meaning of the grading, tribunal barrister Fintan Valentine said.
The letter used in the SB50 meant the source of the information was known to the officer reporting it, Mr McConville said. The number assigned meant the information could not be proved or disproved, not that it was reliable or unreliable.
“It doesn’t necessarily mean it’s true or untrue, it just can’t be tested,” Mr McConville told the inquiry.
The SB50 report also noted that the source of the information was “associated” with PIRA members.
Speaking on behalf of Mr Corrigan, barrister Jim O’Callaghan SC said the grading should be made public, as there was no possible risk to the identity of the source, the late Mr John McAnulty. While the PSNI would neither confirm nor deny that Mr McAnulty was the source, the RUC officer who had received the information had already confirmed to the chairman that Mr McAnulty was the source of the information.
And he argued that information about the grading of Garda security reports had been heard without objection before the chairman, and a different standard could not be applied to the PSNI and An Garda Síochána.
Mr O’Callaghan also said that the PSNI was relying on an exclusion of the public on the grounds of national security under the terms of the Tribunals of Inquiry Act, but only the national security of the Republic of Ireland could be considered by the tribunal, not the security of a foreign jurisdiction.
And he said the PSNI had made no submission about the tribunal on why the document should be redacted.
“The PSNI are keen to maintain the aura of collusion around Mr Corrigan,” the barrister said. But he said PSNI barrister Mr Mark Robinson had never put it to Mr Corrigan that he engaged in collusion during his cross-examination, only going so far as to say the former detective sergeant was well placed to do so.
On behalf of the Garda commissioner, Mr Michael Durack SC said no security issues arose as far as the commissioner was concerned.
Mr Durack said it was already known that the grading system consisted of a letter between A-F, and a number between one and six.
On 11 May, Mr O’Callaghan told the tribunal that his solicitor had “information leading him to believe [the SB50 is] graded C6.”