On July 8th 2017 the on-line edition of the New York Times published Trump Team Met With Lawyer Linked to Kremlin During Campaign
Two weeks after Donald J. Trump clinched the Republican presidential nomination last year, his eldest son arranged a meeting at Trump Tower in Manhattan with a Russian lawyer who has connections to the Kremlin, according to confidential government records described to The New York Times.
The previously unreported meeting was also attended by Mr. Trump’s campaign chairman at the time, Paul J. Manafort, as well as the president’s son-in-law, Jared Kushner, according to interviews and the documents, which were outlined by people familiar with them.
First two paragraph of article, emphasis mine.
Presuming the NY Times is using Confidential  to denote classified information, thus we can speculate that the source was a DOJ document, possibly something from the Mueller investigation, since the other likely source is Jared Kushner’s SF-86, Questionnaire for National Security Positions, which is Personally Identifiable Information (PII) not classified . If it had come from a NSA/FBI intercept the classification would have be much higher. Donald Jr. emails are not classified information, so they are not the source.
Apparently Jared did disclose the meeting, at some point in time, presumably before the NY Time article. It would be informative if when he disclosed this meeting would be revealed. Neither Manafort nor Donald Jr. have a requirement to disclose as they are not government employees nor do they have security clearances. Should the Trump Team have gotten ahead of the “Russia Collusion” story and detailed all meeting with Russian nationals from the time of DJT announcement in 2015, certainly yes. I personally would recommend that his private counsel do this ASAP.
… Kushner’s attorney Jamie Gorelick gave this statement: “As we have previously stated, Mr. Kushner’s SF-86 was prematurely submitted and, among other errors, did not list any contacts with foreign government officials. The next day, Mr. Kushner submitted supplemental information stating that he had had “numerous contacts with foreign officials” about which he would be happy to provide additional information. He has since submitted this information, including that during the campaign and transition, he had over 100 calls or meetings with representatives of more than 20 countries, most of which were during transition. Mr. Kushner has submitted additional updates and included, out of an abundance of caution, this meeting with a Russian person, which he briefly attended at the request of his brother-in-law, Donald Trump Jr. As Mr. Kushner has consistently stated, he is eager to cooperate and share what he knows.”
IMHO the meeting did not fall into the scope of the SF-86 questions, reproduce here:
Note: the SF-86 requires that all questions be answered going back seven years, which is almost impossible to do relative to Section 19 – Foreign Contacts (page 59 of SF-86) for incidental contacts, but incidental contacts are not required to be disclosed:
Do you have, or have you had, close and/or continuing contact with a foreign national within the last seven (7) years with whom you, or your spouse, or cohabitant are bound by affection, influence, common interests, and/or obligation?
NB! A 20 minutes meeting isn’t, by my standards, close or continuing contact.
Section 20A – Foreign Activities (Pg 63 of SF-86)
Have you, your spouse, cohabitant, or dependent children EVER had any foreign financial interests (such as stocks, property, investments, bank accounts, ownership of corporate entities, corporate interests or businesses) in which you or they have direct control or direct ownership? (Exclude financial interests in companies or diversified mutual funds that are publicly traded on a U.S. exchange.)
NB! Certainly doesn’t cover a 20 minute meeting.
Section 20B – Foreign Business, Professional Activities, and Foreign Government Contacts (Pg 72 – 79 of SF-86)
B-1 Have you in the past seven (7) years provided advice or support to any individual associated with a foreign business or other foreign organization that you have not previously listed as a former employer? (Answer “No” if all your advice or support was authorized pursuant to official U.S. Government business.)
B-2 Have you, your spouse, cohabitant, or any member of your immediate family in the past seven (7) years been asked to provide advice or serve as a consultant, even informally, by any foreign government official or agency? (Answer ‘No’ if all the advice or support was authorized pursuant to official U.S. Government business.)
B-3 Has any foreign national in the past seven (7) years offered you a job, asked you to work as a consultant, or consider employment with them?
B-4 Have you in the past seven (7) years been involved in any other type of business venture with a foreign national not described above (own, co-own, serve as business consultant, provide financial support, etc.)?
B-5 Have you in the past seven (7) years attended or participated in any conferences, trade shows, seminars, or meetings outside the U.S.? (Do not include those you attended or participated in on official business for the U.S. government.)
B-6 Have you or any member of your immediate family in the past seven (7) years had any contact with a foreign government, its establishment (such as embassy, consulate, agency, military service, intelligence or security service, etc.) or its representatives, whether inside or outside the U.S.? (Answer ‘No’ if the
contact was for routine visa applications and border crossings related to either official U.S. Government travel or foreign travel on a U.S. passport.)
B-7 Have you in the past seven (7) years sponsored any foreign national to come to the U.S. as a student, for work, or for permanent residence?
B-8 Have you EVER held political office in a foreign country?
NB! Certainly doesn’t cover a 20 minute meeting.
When filling out my SF-86 it was required that all questions be answered, no blanks, using either Not Applicable or None, if no information was provided. And yes, in my case there were several iterations, I would submit and the SSO would come back with questions which would result in a revision and resubmission. The official submission is when the SSO submits it for investigation, a nine month minimum process, back when I submitted my SF-86.
 Confidential is the lowest classification level of information obtained by the government. It is defined as information that would “damage” national security if publicly disclosed, again, without the proper authorization. Part 1, Sec. 1.2, “Executive Order 13526 of December 29, 2009, “Classified National Security Information””. Federal Register – U.S. National Archives and Records Administration, Vol. 75, No. 2, p. 707. January 5, 2010.
 My SF-86 was included in the 2015 Chinese hack of the OPM, so much for the governments protection of PII.