Many websites uses subdomain & cross-domain but didn't know the proper way of the tracking configuration in Google Analytics. Don't continue your mistake and start to fix this at your Google Tag Manager together with Google Analytics now!
The primary reason to configure the above setting is to ensure only 1 user session is counted across multiple domain names.
No more referral domain names under your source which actually is your subdomain or cross-domain.
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https://www.youtube.com/watch?v=XF5V22d-CnE