The Gemora (Bava Kamma 2a) notes: We see regarding Shabbos that there are thirty-nine main categories of melachos (that are forbidden to perform according to Torah law) on Shabbos. This implies that there are sub-categories as well. Regarding Shabbos, we say that the sub-categories are like the main categories. Whether one transgresses a main category or sub-category unwittingly, he must bring a korban chatas. Whether one transgresses a main category or sub-category willfully, he is liable to be stoned. What difference, then, does it make that one is called a main category and one is called a sub-category? The difference is that if one performs two main category prohibitions or two sub-category prohibitions, he is liable twice. However, if he performs a main category prohibition and its sub-category prohibition at the same time, he is only liable for transgressing Shabbos once (and would only bring one korban chatas).
Rashi explains that when one performs an av (main category) together with its toladah (sub-category), he is liable for the av, and not for the toladah. For example, if one planted a tree (av) and watered a plant (a toladah of zore’a), he is liable for the av, and not for the toladah.
The commentators ask: What practical difference does it make if he is liable for the av or the toladah? The bottom line is that he is required to bring one korban chatas!?
Reb Tzvi Pesach Frank suggests the following: The Gemora in Shabbos (71b) rules that if one eats two olive-sized pieces of cheilev (forbidden fats) in one state of unawareness, and he is apprised of the first and he brings a korban. If subsequently, he becomes aware of the second, he is now required to bring another chatas for that one (for the bringing of one korban cannot exempt one from bringing a korban for a violation that he did not know about at the time). Accordingly, if one would perform an av and its toladah together, and he would be apprised of the av, but not the toladah, he would bring a korban for the av. If afterwards he is made aware of the toladah, he would be liable to bring a korban for it, for according to Rashi, one is not liable for a toladah when it is done together with its av.