ANSI Class 150, 300, 400, 600, 900, 1500 and 2500 are the flange pressure rating in accordance with Table 2.1 of API 6D Specification. As outlined above, pressure ratings decrease with rising thermal conditions.
The pressure rating is considered to be the "MOP" or maximum operational pressure for a given piece of equipment. "MTP" is the allowable maximum test pressure, understood in the oil industry to be 50% above MOP.
Since you're probably in the imperal system, for a temperature condition between -20F to 100F, the above specification relative to the given pressure classifications correspond to 275, 720, 960, 1440, 2160, 3600 and 6000 psi respectively.
So now you know.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada