Each task list has a Parent ID field by default, but you can change this and use any other lookup field as the Parent ID. This can be useful if you need to group or categorize tasks in a specific way.
VirtoSoftware uses the Parent ID field for quick boards because the board is designed for a task list. This ensures maximum compatibility with tools like Microsoft Project, Gantt, and similar platforms.
Starting from version 5.2.2, you can also use an external lookup for the Parent ID field.
If your board is created from a custom SharePoint list, it may not contain the Parent ID field (as this is usually included by default for task list functionality). To enable the subtask mode, you must add at least one lookup field to establish parent-child task relationships.
When creating a quick board, the source task list is generated automatically and already includes the Parent ID lookup field required for using the subtask mode.