Only users who have Edit permission can input the values.
On the Issue Screen, Jira has two modes ( Issue Create, Issue View).
In Issue Create mode, Edit permission is required. The users can input the values in the custom field.
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In Issue View mode, the users who have View/ Edit permission will see the value of the custom field.
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Click on this custom field to go to Edit mode.
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If the users don’t have such permission:
Field Security Text ( Single - Line) gets the value from the inputted custom field on the Issue. The application will process the value (encode to store it, decode to show it).