Data retention/ eDiscovery/Compliance basics

Thanks for your response Vasil.

Yes, we can create multiple types of holds in the case however I believe, a hold specific to a case will hold the data present in a users mailbox at that point of time and mails deleted thereafter. 

For a legal matter specific case in eDiscovery which ivestigates historical emails, the holds inside a case won't come handy (except the emails present currently in user's mailbox). In these scenarios, the holds set up initially based on organizations mail retention policy will come to the rescue.

Also, if I choose to setup mailbox in-place hold for all the employees in my company, is the storage space alloted to a user in Office 365 plan used for this or is it a separate chargeable storage (https://technet.microsoft.com/en-in/library/exchange-online-limits.aspx). How does the storage cost work in retention scenarios ?