Hi @jyotisharmav
Apologis for the late reply (been on summer pto).
It's been a while since I've had hands-on for this specific integration. So take the below with a gran of salt, as I would need to test it to fully confirm, but here's my initial thoughts on it.
The limitations of the segment would be that it would only contain profiles that has the ECID set for them, so that they can be recognized by Target. Not all Marketo profiles would have this, as some may be imported from CRM and thereby only have the email and a marketo id set.
In order for them to have the ECID i believe they need have clicked on a link in an email and land on the site - this will ensure that they have the ECID set and that Target can recognize them.
So you will most likely experience that you share a segment of 100 profiles from Marketo, but Target can only recognize and target 20 of them.
Again, I'm not a 100% sure of the above, but this is how i believe it works without having tested it.
I hope this helps. If I find time to test, I'll update this thread with the results. And I'd love to be corrected or confirmed by someone who knows 🙂