The Task Affiliation input type on a custom field can be useful for tracking the item to a specific task.
There is one important factor to keep in mind when working with this field as there can be so many task in a system the field will not show all task across all projects. You will first have to narrow down the possible list before you can use the field. You do this by first creating a Project Affiliation custom field.
Once you have the Project Affiliation field created you can go back to the Task Affiliation field and set the "Parent" to the previously created Project Affiliation custom field. This will make it so once a project is selected form the affiliation list task available to be affiliated within that project will show up in the Task Affiliation field