I, C, Q are additional IMAP functions that are not absolutely necessary to use DC.
But of course it is better if these features are supported by the provider.
I stands for IDLE. This feature allows DC to establish a persistent connection to the server.
Otherwise it must be rebuilt for each synchronization.
C stands for CONDSTORE. This simplifies synchronization and reduces overhead in most cases.
Q stands for QUOTA. If this function is supported, you can display in DC how much memory the account is using. To do this, you have to tap on Chat above the chat list in DC-iOS or on Connectivity in the settings. With DC-Desktop, this function can also be found in the settings under Connectivity.