What does enabling containment on server incur? Having read in a book of Patrick LeBlank "Microsoft® SQL Server® 2012 Step by Step":   


"Benefits and limitations of contained databases    

As with most features, there are benefits and limitations to contained database security. Most of the limitations are specific to containment’s operability with other SQL Server features. If database containment is enabled, you cannot use change data capture, change tracking, and replication"  


I wonder:
does setting "Enable contained Databases" to True on server 



incur limitations on the whole server, the instance of server or specific database created as contained one?  

  

Apparently, it is only to contained database but, then, why it should be enabled server-wide first?
and what does it incur (for example, if I enable it on server but did not enable it on any particular database)?
Instance-wide settings are typical when the database engine can optimize code paths if the setting is off.  In this instance, having contained database authentication = 1 means the database engine must perform checks on each database that it normally wouldn't need to, in addition to checking login requests against any database that has containment enabled.

The real impact on this is probably quite negligible; if you are forecasting that at some near-term future point you might need to enable containment for a specific database, you may consider enabling the instance-wide setting during some maintenance window.