signal contact 記號,唔明

我見到好多人名,藍色字母係人名的第一個字母嘛。對比電話contact list,有人在電話contact不在 signal contact。所以在signal入面見到的人就係有用signal的?那個藍色字母究竟講緊乜嘢?

Other Signal users who are saved to your phone's contact list. When looking at your contact list within Signal, a blue letter in the outside column will indicate it is a Signal contact