I have two cases :
1) I have two site, both sites have different domains ?
(note : we can't go from one domain to another domain )
The ECID will be the same OR different ?
2)I have two site, both sites have different domains ?
(note: we can go from one domain to another domain)
ECID will eb same OR different? do we need cross domain stitching in this case to make ECID same on both the domains ?
In most cases, the ECID will be the same, even without cross domain stitching or the user moving directly from one site to the other.
You can test this with incognito mode... open Site A in a tab, check your ECID. The open a new blank tab, load Site B in that tab, check your ECID there... they should match.
The problem may come if the user loses the third party Demdex cookie between different site visits (expires, user clears cookies, Safari ITP, etc).
I believe that WebSDK does server side cookies which should make this more stable.
Nothing is guaranteed, but for most purposes this should get most scenarios.
What this will not get is the same user between devices. For that you need Device Graph in CJA.
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