Oświadczenie/informacja o zawartości (pojemnika/urny)

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The TSA in the US and the CATSA in Canada likes documentation to accompany the remains, usually a certificate of cremation, a letter of contents, both from a crematorium, and a proof of death certificate. Sometimes an actual medical certificate of death is necessary.
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During the security screening, however, screening officers are not permitted to open the container and will not inspect contents if opened by the individual to which the urn belongs. If it has been X-rayed and the contents can not be determined, it cannot be placed in checked baggage.
https://eirene.ca/blog/cremation-questions-and-answers

There are no alternatives for shipping a body to another country. However, some countries allow family members to travel with cremated remains into the country as long as they carry the pertinent documentation with them. The family must note that if they are travelling on a commercial flight with the remains, they should declare the urn as carry on and present the airline staff with the following:

Proof of Death Certificate
Certificate of Cremation
Letter of Contents (indicating the specified container holding the remains)
Certified Copy of the Medical Certificate Form 16 (if required by the receiving country)

Note that Canadian airlines will not allow carry on transport of metal or marble urns (as they cannot be x-rayed). The receiving country may also have container restrictions.
https://basicfunerals.ca/shipping/

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Note added at 48 mins (2022-08-04 16:20:26 GMT)
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oświadczenie zakładu pogrzebowego na temat zawartości urny

Letter of contents

If you are planning on travelling with the urn and cremated remains (for example, air), a carrier or security checkpoint may request a statement from the funeral home regarding the death, cremation and urn. A Letter of Contents is a letter from us on our letterhead informing the reader of the death, cremation and urn. You would provide this letter to the carrier upon request.
https://justcremation.ca/tsawwassen/services/other-services

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Note added at 52 mins (2022-08-04 16:24:41 GMT)
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Certificate of Cremation/Letter of Contents
Certificate provided by the cemetery indicating the contents of the urn and information of the deceased. This is necessary for interment, shipment, or travel with the cremated remains.
https://basicfunerals.ca/wp-content/uploads/2018/08/Basic-Fu...

Letter of Contents – $20
A letter for transporting cremated remains on an aircraft
https://societysfirst.ca/additional-services/

Oświadczenie (krematorium) o zawartości pojemnika/urny
Letter of Contents
A document signed often by the cremator operator stating what has been placed inside of said urn or container.
https://ecofuneral.ca/91/Glossary-of-Terms.html