The revisions/drafts/history is a feature which allows you to update the Template without changing its published version. This allows you to work on the Template until you are done without worrying that you can break its published version which is used by other users.
The revisions are created automatically every time you update the Template. You can also create a new revision manually.
The revisions have two stats Published and Draft . There can be only one Published revision.
Every time you open a Template in the Template Designer the Latest revision is loaded.
The published revision is used when: Duplicating, creating documents in Flow, showing Template in Templates table.
Add element by double-clicking on it (Elements section)
Delete element by selecting them and press "D" button
Duplicate nodes by selecting them and holding Ctrl/⌥ Option and press "D" button
Resize left(Elements section) or right(Element detail section) by moving the mouse over the gray line between the canvas and the section and hold the left button