Effectively they're the same ...
Published by Mark Hill
Effectively they're the same thing. 'Excess chlorine' refers to the chlorine added after break point has been achieved, where all of the chlorine is in the form of dissociated HOCl, as does residual chlorine. The difference being that the chlorine residual is an analysed value whereas whereas excess chlorine would be a calculated mass value.
Its a little convoluted but you get the gist...