In Shia view, people use (Alayh Al Salam = A.S.) after the name of Imams, but in Sunni view, (Rathi Allahu Anh = R.A.) is used.
What is the reason behind or how do we know which one to use in this website?
Both Sunni and Shia brothers respect a lot to Prophet Muhammad(PBUH), his family and Imams.
قُل لَّا أَسْأَلُكُمْ عَلَيْهِ أَجْرًا إِلَّا الْمَوَدَّةَ فِي الْقُرْبَى - الشوري 23
Both Rathi-Allahu-Anh and Alayh-al-Salam are the signs of being respectful, but even not limited to these.
I believe no matters HOW to be respectful, every sign of being respectful is praised by Allah.