Complex sentence: 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.
Split into simpler sentences: The third and fourth letters are used to form a pronounceable name. These letters are changed when the service number (01 to 99 eastward, 02 to 98 westward) reaches the maximum.