Modify this line as follows. Add the id of the Impact that you want to show. And save it.
<PDM_MACRO name=dtlDropdown hdr="impact" attr=urgency evt="onBlur='manual_modify();detailSyncEditForms(this)'" lookup=no whereclause="id = 1100 or id = 1102 or id=1104">
File menu --> Publish
Run the pdm_webcahce
Delete the browser cache.
Set the configuration of the Priority Matrix (as shown in Figure 2).
The area of the red color square in the following screenshot is the id = 1100, 1102 and 1104.
Although there is the other impact also, since the detail_in.htmpl controls the impact, those 3 impacts only will appear in the dropdown list box of the Impact as shown in Figure 3
The Priority Calculation works with this customization as of today.
Note: This document provided an example of customization. There may be limitations to the support for this customization. It is necessary to check the behavior before implementing in your production system.