“If a sector gets large enough that it has a significant enough market capitalization, like timber REITs or self-storage, that becomes a sector,” Mr. Kuykendall said, rather than falling into the “diversified” class.

Breaking out of the specialty category has been a boon for the self-storage trusts, said Clint Halverson, a senior director of investor relations for Extra Space Storage Inc., a real estate investment trust based in Salt Lake City, said. The four self-storage REITs now have an equity market capitalization of $27 billion.

“Whereas before when we were lumped into specialty,” he said, “you had to market your brand more to investors. It was a little bit more work to get your name out there, and you had to clarify the story and help investors to understand the value proposition in your stock.”

Because REITs that invest in real estate outside the mainstream are often structured differently from traditional ones, financial planners and institutional investors have more research to do in formulating their real estate portfolio strategies. For example, specialty trusts often have an operating business linked with their real estate assets, said Thomas M. Ray, the president and chief executive of the CoreSite Realty Corporation, a REIT that invests in data centers based in Denver.

“Many of the specialty REITs have more of an operational component to them than the traditional four food groups in real estate,” Mr. Ray said. “Because that operating component is so much more important and larger in many of the specialty sectors, then expertise and specialization matter.”

For instance, CoreSite acquires, builds and operates data centers, which house the huge servers necessary to store data for businesses with growing Internet needs. Those unfamiliar with operating these types of businesses might have difficulties predicting the cash-flows of a business doing so.

A lack of consistent and long-term performance measures and the small size of some of the niche markets also can make specialty REITs harder to grasp for investors, he said.