While faithful blog readers know that "source of income" is not one of the seven protected classes in the federal Fair Housing Act ("FHA"), a number of states and localities do include "source of income" as a protected class.  The issue that comes up in these jurisdictions, of course, is just what is covered in the "source of income" of an applicant.  From a legal perspective, the problem is that various jurisdictions differ (or do not always define) just what makes up "source of income."  

For example, in California, "source of income" is defined as "lawful, verifiable income paid directly to a tenant or representative of a tenant."  Connecticut defines "source of income" to include "social security, housing assistance, child support, alimony, public or general assistance."  Philadelphia prohibits discrimination based on "source of income" and defines source of income to include any "lawful income, including all forms of public assistance."  In Chicago, "source of income" is defined to be the "lawful manner in which a person supports himself or his dependants."  As such, it appears that the majority rule is that "source of income" is money paid to an applicant (or a voucher which represents hard money) in whatever form which is to be used for housing.  Perhaps another way to look at the issue is that if the income is reported on a tax return, then it needs to be counted.  To be fair, however, under the law in Chicago, if I was retired but had $1 million in the bank and was going to use that money to pay my rent, management likely would have to (and want to) take me.  Even though I technically had no monthly income (other than perhaps some interest). 
For a professional apartment management company with properties across a number of states, this creates an additional challenge as management likes to use one consistent rule.  However, as is done with occupancy standards, using one firm rule across the country on source of income just does not always work.  You need to know the law in your jurisdiction.  Or reach out to a lawyer like me.
Just A Thought.