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It'll depend on the existing domain that you want to use to host the sub-domain. If the existing domain is already an authoritative domain that has been around for a while and has a high domain authority, it would make more sense to host it under the existing domain as that will enable the new sub-domain to 'share' some of the domain authority. Good way to check is if the existing domain is already ranking for similar keywords that your new Magento store will be targeting with it's products. Hope this helps!

Thanks I'll check it right away.