How to make required System Field optional?

In Defect module there is a Filed called "Feature". It is named as "Parent" and Labeled as "Feature" in Entities>Fields. 

Currently, Feture field in Deefct module is Mandatory field. It has a single dropdown vale as "Backlog". This is "refrence field" type. Im not sure how thsi field is getting a single value as Backlog. If someone can explain this to me wtaht woudl be great. 

Now the issue, Im having here is that, How do I change "feature" field Mandatoryto Optional field?

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