Hi @queirozleonor,
If I'm guessing right, the attributes you're wondering about are INSPIRE GML attributes. You probably won't find many answers here related to the meaning of INSPIRE attributes. However let me try a short explanation:
xsi:nil describes whether an element is nill, in which case it would be "true". The INSPIRE Data Specification states that a nill element must have a nilReason attribute as well, which might have one of the three URI values defined in http://inspire.ec.europa.eu/codelist/VoidReasonValue/
owns is far more complicated and I would just say it is also inherited from the GML specification and should describe the ownership of a referenced element, but bear in mind it is not obligatory and I would strongly recomment just to ignore it. More info in this thread: https://inspire.ec.europa.eu/forum/discussion/view/183049/gml-attributes-owns
Now, if I'm allowed some spamming, if you're writing INSPIRE data, I would strongly recommend trying out con terra's INSPIRE Solution Pack for FME. It helps a lot interpreting all the INSPIRE specifications and includes templates in the FME Hub for most INSPIRE themes with custom transformers which would set up these attributes for you (https://hub.safe.com/?page=1&page_size=10&order=relevance&query=inspire%20solution%20pack). You can download a test version in https://www.con-terra.com/portfolio/inspire-solution-pack-fme or get in contact with con terra at
I hope this helps,
Francisco
It helps a lot, thank you very much!
Best regards,
Leonor Queiroz.