Hello Albert,
You can review the questionnaire in the questionnaire module section at https://www.understandingsociety.ac.uk/documentation/mainstage/dataset-documentation/questionnaire-modules
Here is the link for wave 10: https://www.understandingsociety.ac.uk/documentation/mainstage/dataset-documentation/wave/10/questionnaire-module/scaeureferendum_w10
Although the self-completion EU referendum module was originally intended for Waves 11 and 12, as outlined in the long-term content plan available at https://www.understandingsociety.ac.uk/sites/default/files/downloads/general/long-term-content-plan.pdf, it was ultimately included in Wave 10. However, it's important to note that the module questionnaire wasn't administered during interviews in year 1 sample (2018), accounting for the observed missing observations.
Regarding your second question, at the end of the user guides, specifically, where the content highlights are shown you will find the following information:
*EU Referendum
Wave 12 - * NOTE: Political behaviour content was included for part of Wave 12 to capture the European Elections in May 2019 and the General election in December 2019.
Wave 11 - * NOTE: Political behaviour content was added midway in Wave 11 to capture the European Elections in May 2019 and the General election in December 2019.
These dates correspond to the details mentioned in "This module was included for certain dates only, please see User Guide for details."
I hope this information is helpful.
Best wishes,
Roberto Cavazos
Understanding Society User Support Team