OMAAT only mentions 3 levels for UA (1.5k, 2k and 2.5k), and 3.5k for partners.
I'm not exactly sure what the criteria for 3k is, but IINM, it's on a mixed award (UA and partner) when the first segment is operated by UA, in economy.
For example, a search NYC - TLV for 11/18 bring results in Y for 45k(UA only), 45.5k(UA and OS/LH/LX) and 46k (partner only). In premium and business, it's 3.5k (unless it has to do with saver/non-saver). From TLV to NYC, it's 46k in Y (on a mixed award) even if the trans-Atlantic flight is UA.
Bottom line, this is really simple!