Any orders without member verification will match to existing ACME customers based on name+email address. This is true across all sales channels. In the case of a donation only order placed via Visualforce, an existing contact with an ACME customer ID must be used to complete the order, so the staff completing the order controls the contact/customer that is used.
Any orders placed where member verification was completed (self verified online, or staff transacting as the member through any other channel) will use the customer associated with the primary cardholder. This is true across all ACME Sales channels.
New membership purchases
New memberships purchased online will always create a new customer record in ACME. New memberships purchased via the POS will either create a new customer, or allow staff to choose an existing customer to use as-is for the membership as part of the dupe management flow available to staff. New memberships sold via Visualforce must use an existing contact with an ACME customer ID to complete the order, so the staff completing the order controls the contact/customer that is used.