In the General Use Cases for Profile Merge Rules it states:
Profile Merge Rules options let you expand or tighten audience focus on specific audiences based on business needs or goals. These general use cases explore how to use available options and create merge rules for individual, household, and cross-device targeting. Currently, Profile Merge Rules work with real-time destinations only.
Could you please give me some examples of what a "real time destination" might be? Are S2S destinations considered real time? Would Adobe Target be considered real time? What else?
Among the three type of destinations, URL and cookie destinations are real-time. S2S destinations can be set up to receive data either in "near real-time" (2-min delay) or in batches, depending on the destination.
Adobe Target (integrated via P&A) or legacy method can be considered real-time, along with other cookie or URL destinations.
To summarize, Profile merge rules will work with S2S (real-time), URL & cookie destinations.