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Functionality of this field is very similar to 'Log Work Dialog', you can refer to that section for learning more about how to enter work logs.
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Log Work Custom Field' is provided by WorklogPRO doesn't have work description part. Entered comment to JIRA's own comment field automatically copies to work description. If 'Work Description is required' setting located in the 'General' settings is selected as yes, users can not log work on edit issue/issue transition screens without entering any description to 'comment' field. However, JIRA's own comment field is not present on create issue screen. Since this field is not present on the create issue screen, users can not enter any work description and the error message shows up because of the selection of work description is required. As a result, if the 'work description required' setting is set to yes, users can't be able to create an issue by saving their work on the create issue screen with using 'Log Work Custom Field'.