Apple Watch SE, Series 10 and Ultra 2

Since watchOS and iOS are country specific, not all features might be supported. In case of an Apple Watch 9 or Ultra, the O2 sensor will not work for models sold in the USA, for example.

watchOS - Feature Availability - Apple

iOS and iPadOS - Feature Availability - Apple

The built in hardware might not support all carriers, if it comes to LTE:

Have a look at this article:

Apple Watch - Carriers - Apple

iPhone - Supported 5G and LTE networks - Apple

And the warranty for your Apple Watch may also be restricted to the country of purchase. In case of a warranty claim, you will have to bring it back to the country of purchase. Apple does not ship internationally.

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Apple may restrict warranty service for hardware products to the country where Apple or its Authorized Distributors originally sold the device.

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copied from: Legal - Apple Watch Warranty US - Apple

You're welcome 🙂

I guess, you also saw, that supported LTE Bands differ between US and India Apple Watch models.