Only user users who have Edit permission Edit can choose input the valuevalues.
In On the Issue Screen, Jira have has two mode modes ( Issue Create, Issue View).
In Issue Create mode, Edit permission Edit is required. The user users can choose the values in the custom field.
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Click on this custom field to go to Edit mode.
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If user the users don’t have such permission:
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Field Security User Picker get gets the value from the chosen custom field on the Issue. The application will process the value.