Managed accounts are specific to partnership API access and is a simple means to manage on-boarded accounts by switching them when performing API calls.  The API calls do not change, everything remains the same with the exception of a new variable called ManagedAccountId.  
    
This variable is key and allows you to switch to another account when making API calls.  By switching your account to one you've on-boarded, it grants you access to their website and plugin functionality.