I now have the answer. For anyone else who wants to know how to use this thing, it seems that you have to delete all the values of the pre-filled limits in the additional template, before you enter only the values that you want added to the values of your main template.
If you don't do this, your client who was only allowed upto 15 mailboxes, will suddenly be allowed unlimited mailboxes and pretty much every other value would get messed up too.
Another thing to be careful of is that there appears to be a bug in the applying of the additional template. It can be replicated as follows:
- Create a client.
- Create a template based on the main template, in which 'URL Cron' is selected.
- Save the template and apply it to the new client.
- Create an additional template based on the addon template, where the only value on the form is '1000' in the 'Web Quota' field and where 'URL Cron' is selected (because unfortunately something has to be). Delete all other values on the form.
- Save the addon template.
- Apply the addon template to the new client and save the record.
- Go back and check the client record on the 'Limits' tab and you will see that the cron type has changed to 'Full Cron' when in both templates we were careful to select 'URL Cron'.