During most wars of the 20th and 21st century replacements were sent forward to join units on supply trucks or helicopters. They were rarely sent forward as formed units as they were replacements for personnel lost in formed units along the FEBA. Now Combat units were sent to the Forward Edge of the Battle Area to enhance the combat units already deployed but that was usually when formations were in Movement to Contact and not yet engaged. It all actually depends on the tactical situation as to how personnel get deployed from a staging base.