Exchange Shared mailbox - retention policy for specific

[Taen keren]

I am working for an organization where we are establishing some information governance policies for Microsoft 365 - more specifically EXO.

When a Shared Mailbox is requested/created it gets some kind of "classification" - (I don't know if this 'Classification' is EXO OOTB) - however, I can't see any classification EXO "attribute' possibility in the retention policy in the compliance manager 

We need to apply retention policies to a number of specific shared "classified" mailboxes. 
The company uses an E3 license.

How can this be achieved? 

[#2]

Through Information Governance (aka Data Lifecycle Management), you can add shared mailbox to retention policy:
- Manually for each shared mailbox
- Automatically based on attributes, with Adaptive Scope

Adaptive Scope seems the relevant solution for you. However a E5 license will be needed for each shared mailbox.
Source: https://techcommunity.microsoft.com/t5/security-compliance-and-identity/using-adaptive-policy-scopes...

[#3]

Thank you for your answer

Unfortunately - E5 is not an option :(