Only user have permission Edit can inputted the values.
On In the issue screen Issue Screen, Jira have two mode ( issue create, issue viewIssue Create, Issue View).
In issue create Issue Create mode, permission Edit is required. The user can input the values in the custom field.
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In issue view Issue View mode, the user have permission view or edit will see the value of the custom field.
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Click on this custom field to go Edit mode.
if user don’t have permission:
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