Why the discount is a transfer-tax lever. Gift and estate tax are charged on the value of what was transferred, measured at the moment of transfer. Give away a 20% limited interest in an entity worth $10,000,000 and the tax is charged not on $2,000,000 but on what that particular interest would sell for, which may be materially less. The value that goes untaxed is real to the family, because the recipient still ends up with 20% of whatever the enterprise becomes. A family limited partnership is the commonest vehicle for producing interests with these characteristics, and the entity's own mechanics belong to its own entry.
Section 2703 stops the family from setting the price. For transfer-tax purposes, value is determined without regard to any option, agreement or other right to acquire or use property at less than fair market value, and without regard to any restriction on the right to sell or use it. The exception is narrow and conjunctive: the arrangement must be a bona fide business arrangement, must not be a device to transfer property to family members for less than full and adequate consideration, and must have terms comparable to those of similar arm's length arrangements. All three must be satisfied. A buy-sell agreement among relatives that fixes a low price therefore does not, by itself, fix the value the tax uses.
Section 2704 attacks two specific devices. Where a voting or liquidation right lapses in a corporation or partnership, and the holder's family controls the entity both before and after, the lapse is treated as a transfer by gift or as an inclusion in the gross estate, measured by the drop in value the lapse caused. And where an interest is transferred to a family member in an entity the transferor's family controls, an "applicable restriction" on the entity's ability to liquidate is disregarded in valuing the interest, where the restriction lapses after the transfer or where the family can remove it. Two exceptions survive: a commercially reasonable restriction arising from third-party financing, and a restriction imposed or required by federal or state law.
A discount is an opinion, and the tax code prices a wrong one. Section 6662 imposes a 20% accuracy-related penalty on the portion of an underpayment attributable to a substantial estate or gift tax valuation understatement, which section 6662(g)(1) defines as a reported value that is 65% or less of the correct value. Section 6662(g)(2) applies the penalty only where the attributable portion of the underpayment exceeds $5,000. Where the reported value is 40% or less of the correct one, section 6662(h) makes it a gross valuation misstatement and doubles the penalty to 40%. Two features of that structure matter in practice: the tests compare reported value to correct value, not the discount percentage to some benchmark, and the penalty attaches to an underpayment, so an understatement that consumes exclusion without producing tax produces no penalty either.
What this means for how a discount is supported. Because there is no rate to look up and no safe harbor, the defensible position is an appraisal that identifies the specific interest, the specific restrictions on it, and comparable evidence for each discount claimed. The appraiser qualification rules and the thresholds that trigger them are a separate subject. The practical point is that the discount is the number an examiner will look at first, and the appraisal is the only thing standing behind it.