0 or omitted US (NASD) 30/360
1 Actual/actual
2 Actual/360
3 Actual/365
4 European 30/360

Для 0-го базиса:
U.S. (NASD) method. If the starting date is the 31st of a month, it becomes equal to the 30th of the same month. If the ending date is the 31st of a month and the starting date is earlier than the 30th of a month, the ending date becomes equal to the 1st of the next month, otherwise the ending date becomes equal to the 30th of the same month.

Для 4-го:
European method. Starting dates or ending dates that occur on the 31st of a month become equal to the 30th of the same month.

Остальные используют точное количество дней месяца и точное или приблизительное количество дней в году