To lock a field:
On each field in the forms, it's possible to lock the field for editing by other users.
In the top left corner of the field you can click the 
symbol to lock the field.
You will be prompted to add a mandatory comment to the reason for locking the field.
After adding the comment, press OK.
The lock symbol 
is now red, indicating that the field is locked for editing.
Hovering the mouse cursor over the lock, reveals a hint describing who locked the field: 
.
To unlock a field:
Click on the red lock symbol: 
Click OK to remove the lock.