Most revenues of a company should be coming from core operations. However, it flags up if the major income of a company is generated from its non-operating activities, such as money received from the sale of a few assets, investment income, or some other one-time items. Such an income does not prove tenable and thus can hide the underlying performances of a company.
A company whose major dependence on streams of this nature is to generate income or profit results or simply to avoid net losses can obscure serious issues with its core business. Investors should also be wary of unnatural or odd dependence on non-operating income-they probably only indicate that the company is not earning money within its core business.