The Marchlands is an expansive region of space in the rimward Alpha and Beta Quadrant, stretching from the South-East border of Tholian space to the Klingon Border and below. (See purple area on below picture) It includes features like Delphi Ardu, Cait, Renavi, Armus and the Lembatta Cluster.
Many species inhabit the Marchlands. Among them a large number that has yet been observed much or discovered by Starfleet yet.
In 2394 the USS Constitution-B was temporarily relocated to explore the Marchlands. Their first mission brought them to Starbase 104, which would serve as their new base, far from home during that period of time.