Hi, I would like to seek some advice on what are the differences between Extension of scope & Extension of QS under ISO 17025 requirements.
Apart from that, what are the areas (especially in the QMS) to be take note extending the scope or QS?

I can't help on the differences because I don't recognise the phrase "extension of QS" as having any meaning in the context of ISO 17025, unless it means eg. a major change in top management. Clause 4.2.1 is suitably vague. "The laboratory shall........maintain a management system appropriate to the scope of its activities".

In my experience your Accreditation Body are best to advise on what constitutes an extension of calibration/testing scope or management system change that is notifiable. It might depend on whether you are a calibration or testing laboratory.

This is one of the few areas of ISO 17025 interpretation where ABs in various countries can differ, even if signed up to the international recognition agreements. Some ABs will allow a "flexible" scope which does not meet the published and defined flexible scope requirement of other ABs. To my knowledge it has not caused much concern for the ABs themselves. It tends to be noted only by organisations with international units all trying to get the same operations accredited at the same time with different ABs.