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Inbuilt
Not Add-On
Emphasizes that the 'inbuilt' feature is part of the original design, not an afterthought or upgrade. 
Efficiency Hint
'Inbuilt' often suggests a product or system works more efficiently as it's designed to work seamlessly with other parts. 
Implies Reliability
An 'inbuilt' element is often trusted to be reliable because it is part of the whole structure. 