Only user users who have Edit permission Edit can inputted input the values.
In the Issue Screen, Jira have has two mode ( Issue Create, Issue View).
In Issue Create mode, Edit permission Edit is required. The user users can input the values in the custom field.
...
In Issue View mode, the user users who have View/ Edit permission view or edit will see the value of the custom field.
...
Click on this custom field to go to Edit mode.
...
if user If the users don’t have such permission
Field Security Text ( Single - Line) get 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).
...