... Joomla .ini language files and doesn't have any SEO because you can see it's own output in backend only. Please first read all possible usage cases: https://alterbrains.com/joomla-extensions/administration/multisites-manager/case-studies If you need 3 same websites which have only one different ...

Sounds good and thanks for the speedy response. Given the case study above I'm happy we can have a unique look and feel for teh other sub domains by changing the template file. I'm still a bit curious about how ACL is managed though. You mentioned above ...

... all sites will share same server folder, hence use same files) or not (each slave subdomain will use own subfolder and have own Joomla files)? Our case studies cover all possible usage cases, so if you can't find anything similar - in most cases MightySites won't help you. Please note that MightySites ...

I looked at the case studies and couldn't find one that matched, although as I assume you developed the component you would be able to provide more help. Here's what I think would work: Centralised user credentials and subscriptions ...

... can use same files? Will each site use own database tables or use same tables and have same contents? I strongly recommend to choose a single case study from this page, otherwise we won't have a success: http://alterbrains.com/joomla-extensions/administration/multisites-manager/case-studies

... a subscription component) would be available across all sub domains so long as they were a sub domain of the primary website...? If that's the case then I guess the user records are synchronised across all sub domains...? Would it be possible to also have unique styling on each sub domain then...? ...

... login, so user logged on site A will be also auto-logged on site B. Or it's possible to share session (no need for special single login in this case) if both sites use same TLD - i.e. subdomain.master.com and master.com

That won't work, please choose a valid case study first:
http://alterbrains.com/joomla-extension ... se-studies

Please first choose preferred case study:
http://alterbrains.com/joomla-extension ... se-studies

Better create a subdomain for master site like http://xxx.uco.es
In this case you can add slave sites http://xxx.uco.es/yyy (but not http://xxx.uco.es/yyy/zzz)