It’s different in the sense that zFTP is an estimation coming from your current power curve data analysis (last 60 days). It assumes you have done maximal efforts. If you haven’t, the estimation will not be accurate.
FTP is the value that either you enter manually, because you know the value (field test, laboratory test, etc.) or that the Zwift game returns after you have done a ‘formal’ test (20 min. power or Ramp test). So it’s basically the value that you want defined on your profile, that you assume to be correct.
In case you have done maximal efforts within the last 2 months, in a variety of durations, ranging from 30 seconds to at least 12 min., we encourage the user to set their FTP to the zFTP value.