Hi DatLuu1372,
Before addressing the question, I’d like to share the following information:
Office 365 uses license to control user access to related services. For example, with a SharePoint Online license, users are able to access SharePoint related services. And with an Office 365 ProPlus license, users are able to install Office application to their computers.
In your situation, I’d like to answer your question one by one:
1. Does it require Internet available all the time when using O365? Can I use O365 for PCs that never be allowed to access Internet?
A：Since Office 365 is a cloud product which we need to connect to the cloud to get the related services, in most of the situation, the Internet is required. However, in some specific situation, for example, after we install the Office applications (Excel, Word), it allows the offline works for some time. However, about 1 month, the computer is required to connect to the internet to validate the license for continual usage.
2. According definition from MS, 1 account can be used on 5 different devices. So is it possible to purchase 1 account only (Office product) and distribute it to 5 users? So I don't have to purchase 5 licenses for 5 users
A：From the following article, we can see that we can use one account to install Office applications on five computers with a single Office 365 license. This means the same user can install and use Office on more than one computer, but multiple users cannot use Office from one account. Each user are required to install their own version.
https://technet.microsoft.com/en-us/library/gg702619(v=office.15).aspx
3. Does it require CAL when using some specific products (such as SharePoint, Exchange)?
A：If a user wants to use the SharePoint, Exchange, this user needs to have the SharePoint and Exchange related license to access the services. For example, if you have subscribed to the Office 365 Enterprise E3 plan, it will contains the Exchange, SharePoint, Office applications installation license. Meanwhile, for the SharePoint and Exchange services, it doesn’t need the Office applications every time but with the Office applications, it will make it easy for user to work with them.
If there is any misunderstanding, please feel free to let us know. We will help you further.
Regards,
Charles