Hello @Vinay Negi
there is no really a golden rule for this, but it depends on the features and architecture advantages. Subdomains will make easier to access information accross the main domain (shared folders, application databases) and simplier administration (domain Admins will be able to manage all the subdomain directory).
On the other hand, separated domains will increase security, by segregating services such as DHCP, DNS Zones, etc, but at the same time will increase the complexity of manageability (different Domain Admins or cross domain permissions) and as well access to information (cross domain bindings and permissions)
Hope this helps with your query,
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