Should you choose to associate a new Service Contact you add to a case with with the "Other Service Contacts" option (or the "Other - No Party Association" option), a pop-up notification will appear, indicating that "Parties will be removed if 'Other' is selected."
This notification does not imply that the other Parties will be removed from the case. Rather, it signifies that if you had previously associated this Service Contact information with any other Parties on the case in the Service Contacts tab, this Service Contact entry will now be linked to "Other" instead. If you are adding a new Service Contact and not modifying a previously added one, you may safely disregard this notification and proceed by clicking "OK."