As announced last year (https://techcommunity.microsoft.com/t5/Enterprise-Mobility-Security/Announcing-availability-of-information-protection-capabilities/ba-p/261967), the Azure Information Protection (AIP) Unified Labeling client is now generally available.
This client uses the Office 365 Security and Compliance labels and policies, providing you with new protection features like custom sensitive information type or complex condition.
In the future, all new features will be developed and integrated only on the AIP Unified Client.
If you are only starting using AIP, this is your client of choice.
If you are already using AIP, you can start upgrading your AIP client to AIP Unified Labeling, unless you are using one of the few unsupported features listed https://docs.microsoft.com/en-us/azure/information-protection/rms-client/use-client#features-not-planned-to-be-in-the-azure-information-protection-unified-labeling-client (these will eventually come to the Unified Client). The Unified Client will automatically upgrade your current AIP (either the preview one or the ‘normal’ AIP client).
You can download both the Unified Labeling client and/or the new AIP client from here https://www.microsoft.com/en-us/download/details.aspx?id=53018