The "Choose ARC" option on the registration page (and user "preferences" page for registered users) defines which ARC a user's helpdesk tickets are sent to. The "Choose ARC" drop-down option on the registration page only becomes visible for eligible investigators once they select the country of their institutional affiliation.
Investigators affiliated with an institution in one of the three Executive regions (NA, EU, EA) will automatically have their helpdesk tickets sent to the corresponding ARC, so will not see the "Choose ARC" option. This is a "hardwired" choice and cannot be changed.
Investigators from Taiwan may elect to have their helpdesk tickets sent to either the NA or EA ARC, while investigators affiliated with institutions outside of the three Executive regions (non-ALMA member regions and Chile) may select any of the three Executives for helpdesk support. This is done via the "Choose ARC" drop-down menu at registration, or by editing the user "profile" after logging in to the Science Portal.