Definition
Rule 14e-5 prohibits a bidder, the bidder’s affiliates, the dealer manager, and certain other related parties from purchasing or making arrangements to purchase the target’s covered securities outside the tender offer during the offer period.
Why it matters
Without this rule, a bidder could announce a public tender at one price while quietly buying additional shares in the market at a different price — undermining the all-holders / best-price principle. Rule 14e-5 closes that gap.
Exceptions
The rule includes carefully-defined exceptions: ordinary-course brokerage activity by certain affiliates, specific cross-border situations, and pre-existing trading plans (Rule 10b5-1).
Practical effect
During the 20+ business days the offer is open, the bidder cannot use open-market purchases to accumulate additional shares. The only way to acquire more is through the tender itself (or via the subsequent offering period after expiration).