From Senate Rules Restricting the Content of Conference Reports (Nov. 27, 2017 RS22733):
Two Senate rules affect the authority of conferees to include in their report matter that was not passed by the House or Senate before the conference committee was appointed. Colloquially, such provisions are sometimes said to have been “airdropped” into the conference report. First, Rule XXVIII precludes conference agreements from including policy provisions that were not sufficiently related to either the House or the Senate version of the legislation sent to conference. Such provisions are considered to be “out of scope” under long-standing Senate rules and precedents. Second, Paragraph 8 of Rule XLIV establishes a point of order that can be raised against “new directed spending provisions,” or provisions in a conference report that provide specific items of appropriations or direct spending that were not committed to the conference committee in either the House or Senate versions of the legislation. Both of these restrictions can be enforced on the Senate floor if any Senator chooses to raise a point of order against one or more provisions in a conference report.
The process for disposing of either a Rule XXVIII or a Rule XLIV point of order allows the Senate to strike “out of scope matter” or “new directed spending provisions” from the conference report but agree to the rest of the terms of the compromise. It is not in order, however, for either chamber to alter the text of a conference report, and therefore the process converts the text of the conference compromise minus the “new matter” or “new directed spending provisions” into an amendment.