לע"נ מרת חנה בת ר' יצחק יונה
The mishna in Menachos [22a] teaches that according to R' Yehuda, if the kometz of a mincha became mixed in with a minchas kohen moshiach or a minchas nesachim it is invalid. The reason is that the kometz is a mixture of flour and water that is thick [בלילתו עבה] while the minchas kohen moshiach and minchas nesachim are thinner [בלילתו רכה] and they absorb each from the other.
Rashi [quoted by the Shittah Mekubetzes 2] explains that the kometz is invalidated because it might absorb so much oil from the minchas nesachim that the foreign oil will nullify the kometz [ביטול ברב] so that legally there is no kometz left to burn on the mizbeach. Here is his language:
שנתערב במנחת נסכים פסולה פי' הקומץ של מנחת ישראל, לפי שבלע הקומץ ממנחת נסכים ורבה שמנה של מנחת נסכים על הקומץ ומבטל ליה והוי מנחה שלא נקטר קומצה ופסולה אבל מנחת נסכים כשרה דלא הוי כריבה שמנה כיון דשלא מדעת ערבו בטל וכמאן דליתא דמיא.
It emerges from this explanation that it is only the kometz that becomes invalidated because of the rule of ביטול ברוב while the minchas nesachim remains valid [because there is surely not enough oil in the kometz to invalidate the minchas nesachim]. Since whatever oil flows from the kometz is not desired, it is nullified and viewed as if it doesn't exist.
We see that according to Rashi, the concept of ביטול applies to the concrete מציאות. The excess oil that has been nullfied is כמאן דליתא - as if it doesn't exist. So we see that ביטול is not limited to the realm of איסור והיתר or טומאה וטהרה but affects the actual מציאות of the item in question. However see the Tosfos [ד"ה והן].
