The X-13ARIMA-SEATS Fortran source code is provided by the U.S. Census Bureau and is included in the source package at `tools/x13as_html`.
The x13binary package uses these sources and builds an install-ready binary
package of x13binary. The X-13ARIMA-SEATS manual at

  https://www2.census.gov/software/x-13arima-seats/x13as/windows/documentation/docx13as.pdf

carries thec following statement:

  License Information and Disclaimer (June 12, 2023):

  This Software was created by U.S. Government employees and therefore is not
  subject to copyright in the United States (17 U.S.C. §105). The United
  States/U.S. Department of Commerce (“Commerce”) reserve all rights to seek and
  obtain copyright protection in countries other than the United States. The
  United States/Commerce hereby grant to User a royalty-free, nonexclusive
  license to use, copy, and create derivative works of the Software outside of
  the United States.

  The Software is provided to the User and those who may take by, through or
  under it, “as is,” without any warranty (whether express or implied) or
  representation whatsoever, including but not limited to any warranty of
  merchantability. The Software is taken hereunder without any right to support
  or to any improvements, extensions, or modifications, except as may be agreed
  to separately, in writing, by Commerce.

  User, on behalf of itself and all others who take by, through or under it,
  hereby and forever waives, releases, and discharges the United States/Commerce
  and all its instrumentalities from any and all liabilities and obligations in
  connection with the use, application, sale or conveyance of the Software. User
  shall indemnify and hold harmless the United States/Commerce and its
  instrumentalities from all claims, liabilities, demands, damages, expenses,
  and losses arising from or in connection with User's use, application, sale or
  conveyance of the Software, including those who take by, through or under User
  whether or not User was directly involved. This provision will survive
  termination of this Agreement and will include any and all claims or
  liabilities arising under intellectual property rights, such as patents,
  copyrights, trademarks, and trade secrets. If User of software is an Executive
  Agency of the United States, this clause is not applicable.

  The construction, validity, performance, and effect of this Agreement for all
  purposes will be governed by Federal law of the United States.

  User agrees to make a good faith effort to use the Software in a way that does
  not cause damage, harm, or embarrassment to the United States/Commerce. The
  United States/Commerce expressly reserve all rights and remedies.

The remaining code is copyright by the authors of the R package, and released
under the terms of the GNU General Public License, Version 2 or later
("GPL (>= 2)".

