Strax reports to Sontar that Zygons change their shape to resemble humans, "despite the disgustingly skinny fleshiness that entails." He recounts their previous attempt at conquest, in "Scotchland". He admires this Scotchland — "there, the natives are fierce and take no prisoners."
The designation for this field report has the letter Z for "Zygon" and D for "Doctor". The first 1 in 115 may refer to Queen Elizabeth I, while the 15 may indicate part of the year when a Zygon altercation involving said queen occurs, 1562, as seen in the anniversary special. Alternatively, it could be a combination of "11" as in November and "5" as a skewing of "50", referring to the 50th anniversary on 23 November 2013.