Complex sentence: The Nitrian Desert is sometimes used to mean the entire area region where the monasteries are located, or more specifically it could mean the immediate area around Nitria and Kellia, with the region around Wadi El Natrun sometimes called the Scetis Desert.
Split into simpler sentences: The Nitrian Desert is sometimes used to mean the entire region where the monasteries are located. It can also more specifically refer to the immediate area around Nitria and Kellia, with the region around Wadi El Natrun then more specifically called the Scetis Desert.