In Plesk 10 or newer you're attempting to add a new domain and are unsure how to set them up -- under a new subscription or by adding the domain to an existing subscription.
With Plesk 9 and earlier versions it was a simpler decision in that you could only add new domains since subscriptions did not exist. While the organizational structure of Plesk 10+ does indeed create additional complexity for answering this question, the reorganization of the structure of the panel also allows for some fantastic new features that weren't previously possible in the old structure. This includes the ability to create additional FTP users, add any number of users with Plesk access with granular permissions controls as well as the ability to completely separate groups of domains based on the owner of the domains (separating by client for example).
Problem Resolution
The idea behind subscriptions is that they're meant to be kept entirely separate. The way we try to interpret it is like this:
1. Set up domains under separate subscriptions if they have absolutely no relation -- such as completely separate businesses. This way there can be no confusion about the costs associated with all domains under one subscription and the same goes for user access permissions in the event you need to provide someone specific access to a set of domains owned by the same company.
2. Add domains to an existing subscription if:
a) The domains are connected in some way, such as being owned or operated by the same company
b) There is no chance that they will ever need to be separated in the future, even for allowing others access to individual domains.
As an example, one Websavers Inc subscription would contain some of our domains that represent our brands like websavers.org, websavershosting.ca, grapekiwi.ca, but whenever we work on a new cilent website that isn't one of our properties, we create a new subscription for that client.
