This refers to legislative or judicial acts of States.
"Proving" an Act involves a show of proof that what is presented to the Federal authorities is, in fact, an authentic copy of a legistlative or judicial act of a State. Federal law prescribes the method of proving an Act.
A legislative Act is proved by having the seal of the respective state affixed thereto.
A judicial Act is proved "by the attestation of the clerk, and the seal of the court annexed, if there be a seal, together with a certificate of the judge, chief justice, or presiding magistrate, as the case may be, that the said attestation is in due form."
The Act of May 26, 1790 (1 Stat. 122) states
"Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled,
That the acts of the legislatures of the several states shall be authenticated by having the seal of their respective states affixed thereto:
That the records and judicial proceedings of the courts of any state, shall be proved or admitted in any other court within the United States, by the attestation of the clerk, and the seal of the court annexed, if there be a seal, together with a certificate of the judge, chief justice, or presiding magistrate, as the case may be, that the said attestation is in due form."
Currently it is codified at 28 USC 1738 which states,
"The Acts of the legislature of any State, Territory, or Possession of the United States, or copies thereof, shall be authenticated by affixing the seal of such State, Territory or Possession thereto.
The records and judicial proceedings of any court of any such State, Territory or Possession, or copies thereof, shall be proved or admitted in other courts within the United States and its Territories and Possessions by the attestation of the clerk and seal of the court annexed, if a seal exists, together with a certificate of a judge of the court that the said attestation is in proper form."
See the decision of the U.S. Supreme Court in MILWAUKEE COUNTY v. M.E. WHITE CO., 296 U.S. 268 (1935).
2. The faith and credit required to be given to judgments does not depend on the Constitution alone. Article 4, 1, not only commands that 'full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State' but it adds 'Congress may be general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof.' And Congress has exercised this power, by Act of May 26, 1790, c. 11, 28 U.S.C . 687, 28 USCA 687, which provides the manner of proof of judgments of one state in the courts of another, and specifically directs that judgments 'shall have such faith and credit given to them in every court within the United States as they have by law or usage in the courts of the State from which they are taken.'
And is only necessitated for states remaining in the union, not those whose conventions have rescinded ratification.