When an intervention has already started you can still add maintenance events to it. This way you're able to treat these events or defects during the intervention that's currently in progress, instead of planning them for the next intervention.
To add an event to an intervention that's in progress you first go to the Operational Beta dashboard. Then select your asset from the list (or use the filters at the top of the page to search for the asset) and click on the intervention you wish to add the new maintenance event to.
A form, containing all the information about the intervention, will then pop up. Scroll down until you see a blue "Add event" button and click it. Use the drop-down menu to search the open defects for the one you wish to add to this intervention. (If there are no open events for the asset you'll need to create one first.) Click on the "Save" button to save your changes.
The maintenance event will then appear in the list with the other maintenance events that are part of the intervention.