there are many meanings. what kind of situation is it?

By any chance, did you mean to write "treaty?" A "deal" is a slightly informal term for a negotiated agreement. "The salesperson wanted $26,000 for the car. I offered him $24,000. Finally we made a deal at $25,000." A "treaty" refers specifically to a formal agreement made between two nations. It is usually a long, carefully phrased legal document, negotiated by diplomats, and finally approved by the two governments. A treaty is thus one kind of deal. Treaties may also be called "accords" or "agreements." Wars are often ended when the opposing sides sign a treaty. The United States just negotiated the "United States-Mexico-Canada Agreement." It is a treaty. The treaty concerns tariffs and trade. In the past few decades, there has been a movement toward more free trade, with the result that there have been many such treaties. People who do not like them refer to them as "trade deals," because the word "deal" makes them sound less important.

Are you asking about the case when they have similar meanings? 'Treat' has many meanings, but 'Treat' as a verb is almost never used as a synonym to 'deal'. It has historically been used that way, but if you used it like that these days, you would confuse people. The only word of that family that is commonly used is the noun 'treaty'.