All pages are made up of certain common components. The basic elements of a page object are the page triggers, page properties, controls, control triggers, and control properties.
The following screenshot shows the page triggers. The Help section Page and Action Triggers provide good general guidance to the event which causes each page trigger to fire. Note that the OnQueryClosePage trigger isn't related to any Query object action.
In general, according to best practices, we should minimize the C/AL code placed in Page triggers, putting it in a Table or Field trigger or calling a Codeunit function instead. However, many standard pages include a modest amount of code supporting page specific filter or display functions. When we develop a new page, it's always a good idea to look for similar pages in the standard product and be guided by how that page operates internally. Sometimes special display requirements result in complex code being required within a page...