You can check the columns of any table in the database. (Using pgAdmin i.e.). Or you can check the python code for the _columns attribute. Or you can see all fields of any database object in the webclient under Settings -> Technical -> Databse Structure -> Models. Select the object you want and see the fields-tab. (Technical Features must be activated in the users Access Rights)

I went through the database stucture and wanted to define a rule allowing only tasks assigned to project users to be displayed in list of tasks (project>project> tasks). I thus created a record rule for Project.task with 'apply for read' and 'apply for write'. and the following domain: [('user_id', '=', user.id)] for Project / User group.. It doesn't do what I was expecting... There is something I did not understood

I finally managed to make it by making the rule above as global.