Complex sentence: Common seven-track densities started at 200 BPI sometimes called cpi, then 556, and finally 800, while nine-track tapes had densities of 800 (using NRZI), 1600 (using PE), and 6250 (using GCR).
Split into simpler sentences: Common seven-track densities started at 200 six-bit characters per inch (CPI), then 556, and finally 800. Nine-track tapes had densities of 800 (using NRZI), then 1600 (using PE), and finally 6250 (using GCR).