You can fill in the spare parts needed to perform the tasks of the maintenance plan. This action is only possible when editing the maintenance plan (i.e. after validating its creation, or later).
To do this, simply go to the maintenance plan edition page.
At the bottom of the form is the section dedicated to the spare parts to be released.
Any changes made to a maintenance plan will be made to the tasks in the future, not started, not completed, not cancelled.