Question: Difference in fingerprint appointment scheduling

I have been followin up the issue of fingerprint. After reading a lot posts, I still do not understand the difference in fingerprint appointment made by ourselves according to the I-485 receipt notice and appointment specified by USCIS in its separate note. If they are the same, why there are two options together to confuse people over which one to follow. I and other guys with similar question will appreciate an explanation from experienced people. Thanks.