This new feature allows associating an activity record with multiple related records or related parties.
For E.g. we might want to associate an email with multiple cases, or with case as well as account and contact records.
It is still in the public preview (April 24) and is only available for Email Activity.
To see it in action, first, we need to add the Related field of type Party List to the form.
We can see the option to add related records of type Account, Case, Contact, Knowledge Articles, Queue, and User to it.
Below we can see multiple records of different tables added to it.
Also, we can see the email record showing up in the timeline of the corresponding related records.
Check more details
Hope it helps..
Nishant Rana's Weblog 
Hi Nishantra,
This come additionally to the Regarding fields? or is it an evolution of the regarding field?
Tx
P-O
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Answer found here: https://learn.microsoft.com/en-us/power-apps/maker/data-platform/types-of-entities#enable-users-to-associate-activities-to-multiple-records-preview
Differences between regarding and related columns
While there are similarities between the regarding and related columns, the following are important differences to note:
The related column is currently only available for the Email table. The related column doesn’t currently support the same relationships utilized by the regarding column. The list of supported tables is currently limited to the tables that can have a contact email.
The regarding column acts as a parental relationship from a parent record. For example, if an account has multiple activities and is later deleted, the same action is automatically applied to the child activity records. The related column isn’t a parental relationship. Actions performed on related records don’t affect the related activities.
After the related column is added to the email form, users can manually add one or more records to indicate the records are related to the activity. The regarding and related columns are independent. Setting a value for the regarding lookup column doesn’t affect the values for related. When a new email is related to an existing email (for example, as a reply), the values from the related and regarding columns of the correlated email are copied to the new activity. However, when you create a new email from the timeline control within a record’s context, the related column isn’t automatically set to the current record. This behavior can be modified by enabling an OrgDBOrgSetting named AddParentEntityToRelatedOnNewActivityCreate.
Some features such as Dynamics 365 App for Outlook and Copilot for Sales include an ability to save an Outlook email as an email record while also setting the regarding column. These features don’t set the value of the related column.
Hope this helps 😀
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Thanks for sharing the details. It makes the post complete 🙂
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Hi thanks for your comment. Yes that’s an additional field – Related.
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