Whenever you feel a constant value can change and you can’t add it as a user parameter, store it in ZCONSTS. This table should never be used directly. Instead, a class like ZCL_CONSTS should be created to properly access it, like shown in this article:
http://abapinho.com/2009/06/constantes/ (portuguese only)
Resist the temptation of using T900 or similar tables for this purpose. It’s true that a lot of people do it. But it’s ugly, durty and besides these tables don’t even have an adequate key because they were not designed with this in mind.