Halakhah for Keritot 49:44

The commandment of the sacrifice of an undetermined guilt-offering: That one who is in doubt if he sinned one of the big sins for which a man is liable excision when he does it volitionally and a fixed sin-offering when inadvertent, bring a sacrifice (Keritot 25a). And the doubt emerges for him in this way: As you might say, by way of an archetype, that there were two pieces in front of him - one of forbidden fat and one of permissible fat - and he ate one of them and the other got lost. And [so] he worries to himself as he does not know if he ate the one of forbidden fat or the one of permissible fat. This sacrifice that is brought upon this doubt is called an undetermined guilt-offering. And the word undetermined (talui) is said about anything about which it is fitting that a matter come afterwards that reveals about it that which was not known previously. For example, if [one] knows that the sinner ate forbidden fat - behold, it is revealed that the first [sacrifice] was not sufficient for him and he needs to still bring another sacrifice that is called a fixed sin-offering to complete his atonement; but if one knows about him that he ate what was permissible - behold it is revealed that the first was sufficient and he does not need to bring another sacrifice after it. This is the explanation of its being undetermined. And the command for this sacrifice is from that which it is written (Leviticus 5:17-18), "And if a soul sins and does one of all of the commandments of the Lord which you shall not do and he does not know, but he is guilty and he shall bear his iniquity. And he shall bring an unblemished ram from the flock, according to your assessment for a guilt-offering to the priest, and the priest shall atone for his inadvertent transgression which he transgressed and did not know" - meaning to say, about his not knowing if he transgressed inadvertently or [not]. And the Sages called this matter, "not known."