But bit by bit, especially in recent years, that has changed as the I.R.S., in a number of low-profile decisions, has broadened the definition of real estate, and allowed companies to split off parts of their business that are unrelated to real estate.

For example, prison companies like the Corrections Corporation and the Geo Group successfully argued that the money they collect from governments for holding prisoners is essentially rent. Companies that operate cellphone towers have said that the towers themselves are real estate.

The conversions generally do not require the companies to change their underlying business. The chief executive of the Corrections Corporation, Damon T. Hininger, told investors in February that the new structure should help in the company’s aim of “housing more and more population for federal, state and local levels as they grow or deal with overcrowding.”

The I.R.S. released its latest decision, allowing a data and document storage company to convert, on April 5. The letter did not include the name of the company, but several data storage companies, including Iron Mountain and Equinix, are in the process of converting.

A few days later, a strategist at the Wall Street firm Jefferies wrote in a report: “It is not a far stretch to envision REITs concentrated in railroads, highways, mines, landfills, vineyards, farmland or any other ‘immovable’ structure that generates revenues.”

Today, there are more than 1,000 real estate investment trusts, about 10 percent of them traded publicly on the stock market. Investors like them because, by law, they must distribute at least 90 percent of their taxable income to their shareholders — a particularly alluring prospect today, given the low interest rates paid by many other basic investments.

The benefits of converting are obvious for stockholders and corporate insiders as well. The conversion typically drives up a company’s stock price. Investors are drawn by the prospect of lucrative dividends under the new structure. The mere rumor that a company might convert has been enough to send its stock price soaring.