Be chary of using this word in its new-fangled sense of class or category. About this Ivor Brown says:
The poor used to be called the poor; then they became, lest accuracy offend them, the under-privileged, or Lower Income Groups. Recently they have been called, especially by economists aiming at style, the Lower Income Brackets. I suppose the reference is to types and species bracketed together. But the usage is a stupid one. Somebody employed the term, I suppose, in an impressive article and so all the impressed readers decided to pay him the compliment of imitation.
See also Group.
For brackets in the usual sense, see Parenthesis.