Subprojects and milestones are now available in the UI, but still rough.
Subprojects and Milestones
Projects now have a "Subprojects" tab, which allows you to create and review subprojects and milestones. These features generally have good infrastructure and backend support today, but are still missing a lot of UI support. I expect them to be difficult to use until the UI support features are more fleshed out.
The new documentation (Projects User Guide) does a reasonable job of describing these features.
For now, when you create a milestone, a column for it will automatically appear on the parent workboard:
Dragging cards between columns on the parent workboard automatically adds and removes the appropriate project tags.
For example, dragging a card from the Stoneworks → Iteration II milestone column to the Stoneworks → Iteration III milestone column will remove the Iteration II tag and add Iteration III. Dragging a card to the Backlog column (or any other normal column) will remove any milestone tag and replace it with the parent Stoneworks tag.
This allows you to use the parent workboard as a higher-level strategic overview for planning across milestones. The milestone projects themselves have their own workboards which show only the contents of that particular milestone, and can be used more tactically.
Here are the same cards in a more detailed view on the milestone workboard:
This is the general shape of how Subprojects and Milestones will work, although many UI features and integrations are still untouched or rough. If you want to begin testing these features you probably won't break anything, but they may still be too clunky to help with planning and organization very much.