"I was told by an I.R. person at another company that it can't be known until end of year what the tax treatment will be"
Determination for treatment of distributions is an accounting determination. It is not something preplanned or decided by a board of directors. If a distribution is not covered by earnings and profit (E&P) (either past (accumulated) or present) it is treated as a return of capital (ROC). However, management or the board of directors can make decisions to purposely affect accumulated earnings in an attempt to influence distribution determination. A return of capital is a return of some or all of your investment in the company. There are instances when assets have been depreciated (accounting) that it is advantageous to return the investment capital to shareowners or partners. Certainly it is easy to envision why MLPs depreciate or would have accumulated (retained) earnings. Obviously with an equity (property) REIT depreciating assets is commonplace. Off hand I cannot think of how a strict mREIT would have depreciable assets. Typically with REITs ROC occurs when a company must pay a required dividend but earnings make it unable to do so from its profits..