What governing factor determines if an individual from a non-partner nation can fly to the ISS as an 'Invited Guest'?

Answer

Bilateral/Agency Agreements, usually brokered by ESA or JAXA

Individuals from non-partner nations fly usually under agreements brokered by a partner agency, allowing them to fly under an umbrella agreement.

What governing factor determines if an individual from a non-partner nation can fly to the ISS as an 'Invited Guest'?

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