The third and fourth letters are used to form a pronounceable name, changed when the service number (odd 01-99 eastward, even 02-98 westward) reaches the maximum.
The third and fourth letters are used to form a pronounceable name. A pronounceable name is changed. The service number (odd 01-99 eastward) reaches the maximum. Odd 01-99 eastward are even 02-98 westward.