Requesting a certificate of residence to prevent double taxation UK and Romania- confusing form

Hi, I am self employed in the UK, and going to Romania to hold a workshop at the festival there. They will pay the taxes in Romania for my pay - so I will only receive the amount after Romanian tax is deducted. They are asking me to provide the Certificate of Residence to prevent a double taxation. but I am confused which boxes to tick: 1. Which article of the country's Double Taxation Agreement covers the income in this request? Have no idea which one applies to UK -Romania work 2. Do the subject to tax provisions apply to this foreign income? (the tax on this income that I receive while being in Romania is paid by Romania on their side) Could you please help?

Hi 

In answer to your questions:

1. Article 4 for residence  

2. no

Thank you