Employees are not allowed to edit their own person-id-external hence, you set the person-id-external field as View only from the data model:
"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."
This would set the field as read only / view only in an Employee Self Service scenario:
Try to add a new employee from Add new employee wizard: person-id-external would be set to read only / view only as well.
Employee Central v2.0
Reproducing the Issue
1. Set person-id-external visibility to view only
2. Go to an employee an edit Biographical Information - Person ID field shouldn't be editable
3. Add a new employee from Add new employee wizard - Person ID field is also not editable (you want to set this as editable from new hire screen)
RBP Constraint for Biographical Information portlet, fields aren't permissible individually:
Create an onInit business rule using Employee Information Model as it's base object that sets the visibility of the person-id-external field to "both", assign this rule to personInfo HRIS Element. This should only trigger from New hire / Add new employee screen:
Result (Add New Employee Screen):
person-id-external editable new hire editable Person ID field in new hire business rule set person-id-external as editable business rule set person ID editable both , KBA , LOD-SF-EC-PER , Person Data (All Person Data Types) , LOD-SF-EC-RUL , DO NOT USE - Business Rules & Event Derivation , How To