What is the difference between welcome notice and approval notic

Hi, Xiaobaitu,Pijiang and experts,

I received two seperate emails from USCIS. The first email mentioned to send welcome notice on Oct.7. Another email was sent on Oct.21. This email mentioned to send I-485 approval notice on Oct.10.I already received welcome notice and GC but I haven't got any I-485 approval notices. I get confused about this. Can anyone tell me what is the difference between welcome notice and approval notice? Thanks!