By default, External Sharing viewers may not be able to see all of the Taskfeed data which is visible to your internal users, such as Checklist items or custom fields. However, Salesforce System Administrators can configure field-level visibility for the External User Profile.
|Salesforce User Permissions Required|
|To create and edit Force.com sites:||Customize Application|
Configure Field-Level Visibility for External Sharing Viewers:
Once External Sharing is enabled for your Org, go to Salesforce Setup and search for Sites in the Quick Find search bar.
Select Taskfeed Public Share Services
Click Public Access Settings
Scroll down to the Custom Object Permissions section
Check the box for Read in Checklist Item
You can now customize further what the external user can see in the Field Level Security section of its profile. For instance, you may want to display the Details field or a custom field you may have within Taskfeed Tasks.
Other use cases:
Use the same steps above to make a Taskfeed Task custom field, such as Notes (rich text area that can contain URLs and images) visible in External Sharing.