Two International Place in Boston has a mix of direct, sublease and shadow space, which is defined as space that is technically leased to a tenant but is currently unoccupied.

It’s been called the wild card in commercial real estate; a factor whose effect on market recovery is yet to be known. Shadow space, a fairly new phenomenon after the late 1990s real estate boom, in many ways remains as mysterious as its name might imply. While some brokerage houses peg the number at around 5 percent of all office inventory, others speculate that it could be as high as 10 percent.

Shadow space is the term for commercial space that is under lease but, for any number of reasons, is nevertheless not being used by the tenant. The problem with shadow space, researchers say, is that you can’t easily track it. The unused portion of a building often is visible only to those who happen to see a locked door or empty rooms behind windows.

“Companies took more space than they needed when the economy was growing and companies were growing,” said Gilbert B. Dailey, senior director of the Boston office of Cushman & Wakefield. “Some have subleased while others have decided to hold onto it.”

So when is it economical to transfer excess space into subleased space? Industry experts point to a host of situations, although many also point out that new accounting guidelines exist that discourage the practice.

Traditionally, companies wanting to rid themselves of all or part of their office space were allowed to defer the recognition of a discontinued asset until a subtenant was found. That grace period no longer exists. Following the accounting change in January 2003, the minute companies decide they want to shed excess space, they have to take a loss on the books equivalent to the remaining term of the lease.

Brendan L. Carroll, research analyst for Grubb & Ellis Co., offered as an example a company halfway through a $1 million-per-year, decade-long lease that decides it wants to market the space to a subtenant.

“That company takes a $5 million hit today,” Carroll said of the accounting process. “Companies must weigh this in deciding whether or not to sublease their shadow space.”

William P. Barrack, principal of the Boston office of Spaulding & Slye Colliers, said he knows of several companies unwilling to dump shadow space because of the accounting guidelines.

“We need job growth. All this shadow space means that it will take longer to recover,” he said.

For some companies it may be a marketability issue. The space may be an odd shape or in an undesirable location, or it may just not be worth the inconvenience to a company with only two years left on its lease. And with scads of space available at present, finding a subtenant often is no easy task.

“In certain markets, where there’s more than a 30 percent vacancy rate, a large company may say, ‘we’re not going to deal with the headache and we’re just going to let it sit,'” Carroll said.

Others mentioned the pure economics of the decision-making process, which often precludes companies from putting space on the market.

“Oftentimes they’re not prepared to suffer the consequences of reducing rent because they may be at a high rent. For example, if they’re at $35 and the market’s [at] $18, perhaps it’s too much pain. However, my view is that the enemy of a good deal is a better deal and I would take the $18 if only to bridge me through this period,” said Mark S. Schuster, president of Bluestone Holdings, based in Newton. “… Oftentimes many things are going on behind the scenes that aren’t obvious to those of us not in the company, i.e. expansion and growth plans, plans to sell or other such things.”

Jack Burns, principal of Boston-based CRESA Partners, has seen companies at which 40 percent of it’s leased space is shadow space, and while to some it would make sense to sublease the unused portion, company executives see the business disruption as unfavorable, deciding instead to wait it out.

“It’s like the camel standing on the table, no one notices it until one day…,” he said. “That doesn’t change until CEOs and senior managers say, ‘we have to save money, let’s see what we can do about it.'”

Burns said a threshold of 20 percent or more of excess space is a good number at which firm’s should seriously look at consolidation and sublease, assuming that there’s a reasonable amount of time left on the lease. Once the decision has been made, the transfer can be complicated. Take, for example, a company with a 50,000-square-foot lease and a 15 percent empty seat ratio spread over five floors. It’s not concentrated, and therefore, not marketable. The company can consolidate, freeing up the first floor, but that will affect every department. And many companies like to keep their departments in distinctly separate areas, Burns said. The physical disruptions may not be worth it.

“It’s a gut feel,” he said. “There’s no scientific formula” to determine when it is worthwhile for a company to attempt to convert shadow space into sublease space.

The wild card of shadow space complicates projections for recovery. Joseph P. Fallon, principal of the Boston office of the Trammell Crow Co. said that the real challenge is determining how much excess space should remain for internal growth and how much should be released into the market as subleased space.

“Neither one is great for owners of real estate,” he said.

Claude Hoopes, principal of the Boston office of CRESA Partners, said there are several Boston vacancy phenomenons; the question is whether these factors will improve within the next six months.

Hoopes said that a flight to quality has bifurcated the Boston market. Availability rates of Class A space above the 20th floor remains at only 5.5 percent to 6 percent. Below the choice spots above 20th floors, availability has risen to over 20 percent.

“The other concern is that there’s a national buzz that the economy is recovering but in Massachusetts, two out of every 10 jobs created is going offshore,” he said.

Losing jobs overseas, consolidation within the financial services industry and other economic trouble spots have left many wondering exactly when the commercial real estate market will recover. Shadow space, industry watchers concede, makes any prediction difficult at best.

“All of that means that the recovery and health of the office market will be extended. It could easily be into 2006 before we have enough vitality and job creation before we have positive absorption,” said Hoopes.