Flat 6 have three cylinders on each side with a 180 degree sepetation like a scooby has a flat 4 engine.
A straight 6 has 6 cylinders in a line like in most modern run arounds cars have a straight 4 set up M3's have straight six engines, so do some Jags and other cars i dunno. Although the new M£ to be realesed is coming witha a V8