By default the user of a client can edit his sites but cannot access to options tab in site configuration. Is there a way to allow this? Thanks a lot.
No, if it is accessible to the client, then the client can potentially take over sites of other clients or even the whole server. So there is no way that a client can access these security-sensitive settings.
What you can do, though, if you need specific configurations for some cms systems, create them under System > directive snippets. The client then has a select field on the first tab of website settings where he can select one of these config sets.
Thanks till! The problem is that what I want is to provide the client with control to the max upload limits. So it becomes a bit more difficult to achieve... I would have to provide several preconfigured limits, but they would not be able to set their own limit. By the way, I am a bit in a mess on how the combo that selects configuration works. If I select a snipet in this combo... it ignores nginx configuration box in option tab? (that is what it seems to me). Thanks again, I appreciate a lot your help.