That's a tax question. At the present time, it means you pay taxes on the dividends at your marginal income tax rate, not the 15% rate that applies to "qualified" dividends.  REIT dividends are not "qualified."

If you held O in a tax-sheltered account then you would pay no taxes on the dividends until some time in the future when you are withdrawing from the retirement account.

Of course, the treatment in a taxable account could change over the next couple of years at the whim of congress.