Once you file an I-526 petition, and if the I-526 petition is properly filed (meaning, forms are properly filled out and signed, filing fee check included with correct amount, and includes all evidence necessary to adjudicate the case), USCIS will cash the filing fee check and issue the I-797C receipt confirming they received the case and assign you a priority date. If it isn’t accepted or deficient in any way, USCIS will return the entire case along with a note explaining why the case it was rejected and it must be cured before refiling and acceptance.