Every group will have someone listed as something like main vocal, lead vocal, power vocal, romantic vocal, or emotional vocal. However, many idols are simply listed as vocals with no preceding title, meaning they sing as opposed to rapping, but they usually get fewer lines in songs than vocals with specific titles.
These idols are singers because after all, idol groups are singing groups, but they usually contribute to the group in bigger ways offstage. Many are really good at growing the fan base through fan service. Others may be great on variety shows or host on music shows, gaining notoriety for the group. Some focus on acting between promotions. Groups usually have one or two members who place a special focus on foreign languages in order to better communicate for the group in overseas promotions.
When an idol introduces her/himself simply as a vocal, that means you probably won’t hear them belting long notes, rapping, or showing off in dance breaks, but look for them to excel offstage in one of the countless ways idols reach out to the fans.