The estate tax definition, in full. Section 6166(b)(1) reaches (A) an interest as a proprietor in a trade or business run as a proprietorship; (B) a partnership interest in a partnership carrying on a trade or business, if 20 percent or more of its total capital interest is in the gross estate or it had 45 or fewer partners; and (C) stock in a corporation carrying on a trade or business, if 20 percent or more in value of its voting stock is in the gross estate or it had 45 or fewer shareholders. Section 6166(b)(2) sets the counting rules: the test is made immediately before death, an interest held by a married couple as community property or as joint tenants, tenants by the entirety or tenants in common counts as one owner, property held through a corporation, partnership, estate or trust is treated as owned proportionately by its owners, and stock or partnership interests held by members of the decedent's family are treated as owned by the decedent. The value of passive assets held by the business is excluded both when the estate tests whether the interest is large enough to qualify and in figuring how much tax can be deferred. What the executor can then do with a qualifying interest belongs to estate liquidity planning.
The income tax definition. The passive activity loss rules apply to individuals, estates and trusts, and to two kinds of corporation: personal service corporations and closely held C corporations. The at-risk rules of section 465 reach the same closely held C corporations. A closely held C corporation that is not a personal service corporation gets one concession under section 469(e)(2): its passive activity losses may be deducted against its net active income, though not against portfolio income such as interest and dividends. The same five-or-fewer test is half of the definition of a personal holding company, whose undistributed personal holding company income bears a separate tax, and it is the test a real estate investment trust must fail, since a REIT may not be closely held.
Valuation without a market. A public company's shares have a price every trading day. A closely held interest has to be valued by appraisal, for estate and gift tax, for a buyout and in a divorce, and a minority interest that carries no control and cannot easily be sold is generally worth less than its proportional share of the whole business. That gap is the subject of valuation discounts, and it is one reason estate plans for business owners use buy-sell agreements to fix a price and a buyer in advance.
Pay and distributions. When the owners of a closely held C corporation also work in it, they have a reason to call profit compensation, because salary is deductible to the corporation and dividends are not. Whether that pay is reasonable for the work done is the question the reasonable compensation rules answer.