Various standards that are established by stock exchanges (such as the NYSE) to control membership in the exchange. Companies wishing to issue their stock on a given exchange must meet its listing requirements and continue to do so for as long as they are on the exchange.
Investopedia Says:
While the particulars vary by exchange, the two most important categories of requirements deal with the size of the company (as defined by annual income or market capitalization) and the liquidity of the shares (a certain number of shares must already have been issued).
For example, one of the listing requirements in the NYSE for public companies is that the company must have at least 1.1 million publicly-traded shares outstanding that are worth at least $100 million.
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