Every change to an item is saved as a new version. Nothing is lost when you or a colleague edits an item: you can look at any earlier version, compare two versions attribute by attribute, and go back to one.
Opening the version timeline
Open the item details sidebar and click the clock button (History) at the top right. The timeline lists the versions newest first. The version you are looking at is marked current in bold; while you look at an older version, the top entry is marked latest instead. Each entry shows how many changes it contains, who made them, and when.
Viewing an earlier version
Click a version in the timeline. The item in the main view switches to that version, read-only, with a header reading Time machine item version N of M. Click the X in that header, or click the current version in the timeline, to return to the live item.
Comparing two versions
Click the ... button on a version in the timeline and choose Compare with current item version. The Version history dialog shows the two versions side by side:
Two dropdowns at the top let you change either side to any version, labeled Version N (current) or (latest) with its date.
The heading and description are shown with additions, deletions and formatting or value changes highlighted; the legend at the top right explains the colors. Switch Highlight changes off to read the plain text.
Attributes lists every other attribute, with added or deleted ones highlighted like text changes.
Traced to lists the traces at each version and marks traces that were added or removed between them.
View comments at the bottom of each side opens the comments made on that version.
Reverting to a version
Click the ... button on the version you want back and choose Revert to version N. Trace.Space creates a new version whose content matches the chosen one, so the history stays intact and you can revert again if needed. You need write access to the item.
Versions in baselines, reviews and test runs
Baselines, reviews and test runs hold items at a fixed version. Their item sidebars have the same History timeline, with the pinned version marked current baseline (or the review or test run equivalent), and the ... menu offers Compare with current baseline version, Compare with current review version or Compare with current test run version.
For a baseline that is not frozen, the menu also offers Change baseline item to vN, which repins the baseline to that version without touching the live item.
When a variant is in scope, the timeline marks which version that variant pins. A pin is either a fixed version, which stays put as the item moves on, or a live one, which follows the latest version. A pin that has fallen behind can be bumped to the latest version in one action. See Viewing and managing baselines.