The Notitia Dignitatum, (translatable as "notable worthies" or "record of offices") was an official register of all Roman civil and military posts in two parts, the first in partibus occidentis dealing with the Western Empire, and the second in partibus orientis, naturally enough for the Eastern Empire.
It has survived in the form of four manuscripts, now located at Oxford, Paris, Vienna and Munich that are exact copies of the eleventh century (but now lost), Codex Spirensis, itself believed to be a copy of the original
Its exact dating is uncertain and disputed, but the consensus appears to be that it was first drawn up between the years 395 and 408, with later revisions up to 425. (Many of which are believed to be of questionable accuracy.) Its importance lies in the fact that it gives us a detailed, if incomplete picture of the nature and extent of the Roman bureaucracy as it stood at early in the fifth century AD.
As regards Roman Britain, it tells us of the existence of the Saxon Shore defenses, the commands of the dux Britanniarum and the comes Britanniarum, as well as detailing the civil administration of the diocese of Britain.
SOURCES
An Age of Tyrants - CA Snyder - Penn State Press (1998)
Roman Britain - P Salway - OUP (1991)
www.fordham.edu/halsall/source/notitiadignitatum.html which has an incomplete translation.