How Does Truncation of Identity Numbers Work

How does truncation of identity numbers work?

A) Machines encode account numbers when transmitting them over the internet.
B) Machines can only print the last 5 digits of an account number on anything that is printed for mailing.
C) Clerks and telephone operators are only allowed to verify identity by the last five letters of a persons name.
D) Older machines that do manual imprinting of credit card numbers must be modified to only copy the last five digits of the account numbers.