Substitute Civil War for 102nd Infantry out of Allegheny PA

In register my ggg grandfather David C Booth shows up as a substitute and has as his alias John Jay Booth. He was in from August 22,1864 - June 20, 1865 and received a pension. He was in the 102nd Infantry out of Allegheny PA. What was the deal about the alias and being a substitute. Did he substitute for John Jay Booth? I am trying to find his parents - his father came from England before 1825. David spent most of his life around Ohio.