customer service为什么有时候需要加s

customer service为什么有时候需要加s customer's service department是什么意思 our有时候为什么加s Customer Service customer service representative是什么意思 Free Customer Service Phone customer service是什么意思 Customer Service for Warehouse customer service officer是什么意思 what is Customer Service? Customer Service什么意思 as per customer instruction,这里customer要加's吧?