Because British Land is a REIT, it must follow certain rules relating to money it distributes to shareholders, and how those distributions are taxed.
90% of the tax-exempt profit from British Land's property rental business has to be distributed to shareholders. This is called Property Income Distribution, or 'PID'. British Land can also distribute income from its other activities, known as Non-Property Income Distribution, or 'non-PID'.
Dividends can be entirely PID, entirely non-PID, or a split between the two; the Board will decide the most appropriate split on a dividend-by-dividend basis. Sometimes the treatment of the Scrip Dividend Alternative may be different to the treatment of the cash dividend.
The tax treatment of PID and non-PID dividends is different. Non-PID dividends are treated like "normal" dividends paid by non-REIT companies. From 6 April 2016 the tax free Dividend Allowance will apply to non-PID dividends. Previously a tax credit of 10% was deemed to be deducted from the non-PID dividend payment.
Because PID dividends are paid out of British Land's tax-exempt profits, they are potentially taxable in the shareholders' hands as property letting income. PID dividends are usually paid to shareholders after the deduction of withholding tax at the basic rate of income tax (20%).
More detailed information can be found here and on the Reita website.