[Custom Field] Transaction Body Field Team, What does the highlighted field mean? My Customer wants to use the Type field for the Transaction. But I cannot find what means this field when the user selects Multiple Select. And how this field should work with this value? Could you please provide a broad explanation?

This field will be a drop down list that the user must select from a list of predefined values. The List/Record field right below it is the list or record that is sourcing the predefined values Because you set it to Multiple Select the user can select multiple values from the list instead of being limited to just one selection

AI1’s answer is right. Here’s an example of how I use mine. I need a multiple select field on a task in the Related Records subtab to pull a list of lot numbers associated with the item on the task. The multiple select field uses the Inventory detail record to create the list and on the sourcing tab, it pulls only the lot numbers that match the item in the filter.