By default a custom field value can move freely between any of its options. Where a field records an outcome rather than a preference, some of those moves destroy the record of what happened: a failed inspection becomes a passed one, a rejected application becomes an approved one.
Blocked transitions make those moves impossible instead of merely discouraged. A common setup is to allow Fail to Resolved but never Fail to Pass, so the history of the failure survives.
Where to find it
Go to Processes → select your process → Settings → Custom Fields. Click edit on the field you want to control.
In the Edit Custom Field modal, find the Blocked transitions section. Each of the field's options gets its own dropdown.
Blocking a transition
Open the custom field for editing.
Under Blocked transitions, find the row for the value you are moving from.
In that row's dropdown, select the values it must not move to. Leave it on No restrictions to allow every move from that value.
To stop someone emptying the field from a given value, select (cleared) in that value's dropdown.
Save the field.
Example
On a Lead Safety Result field with options Pass, Fail, and Resolved, set the Fail dropdown to block Pass. Fail can then move to Resolved, but never straight to Pass.
Good to know
Rules are directional. Blocking Pass to Fail does not block Fail to Pass. Set each direction you care about.
A blocked move is refused through every route, including the UI, API, bulk edit, import, integration sync, and automation, with a message explaining which move is blocked.
Allowed transitions are unaffected and need no extra confirmation.
Values already set when you add the rules are not changed retroactively. Future moves from those values follow the rules.
Fields with no transition rules behave exactly as they did before.
Blocked transitions sit in the same modal as restricted options, and the two work together: a move can be blocked outright, or allowed only to users holding the Set restricted field options permission.