The sole source of data published by ENTSO-E, are member TSOs. Networks of data correspondents are nominated for each specific report or for each specific purpose of data delivery.
To report on those cases where the situation of a given country does not match the definitions provided on the document Guidelines for Monthly Statistics Data Collection and on the ENTSO-E Metadata Repository, a document on specific national considerations is made available.
This includes specifications relating to the geographical perimeter (when data is collected on a perimeter that does not exactly correspond to the perimeter of the country), representativity (when the data delivered does not represent 100% of a given geographical perimeter, but only a part of it), and other aspects concerning quality, availability and interpretation of data.
The document is updated on a regular basis, the last version dates of 14 September 2016 and can be viewed from the link below: