You can customize the application form for every different job opening, and decide which of the fields are required. Here's how to do that:
Marking fields in the application form as required
From anywhere in the tool, click Jobs in the sidebar
Click on the job you would like to adjust the application form for. Click on Apply form in the job navigation at the top of your screen
Click the Edit icon behind a question
Next, switch the toggle at "Require an answer" to mark the question as required
Click Apply changes in the bottom right corner
Then, click Save changes in the top right corner.
Note: Any changes made are only for the application form of this job opening