Piggy-back applies to contractual agreements in law, more specifically shareholder selling rights.
To apply, a piggy-back clause must be included in a corporation's shareholder agreement, which is part of the incorporation materials.
Because the shareholder's agreement is a contract, the rules are rather soft, and a piggyback clause can be tailored to fit the specific needs of the company.
Generally, a piggyback clause applies only to a majority shareholder or someone with a large portion of the shares.
However, they can be a benefit for larger communities by saving administrative costs and by creating pressure for lower prices.