to answer your question Torvy, all new 2021-2022+ models with an hst trans will have the hydromax feature, on the older 20 and 15series it was not standard on the hydrostats some had them some didnt, like our 3520h that belongs to the shop while it is an hst it does not have the hyrdomax feature.
as for what hydromax is, basically its the system branson uses to tie the movement of the forward and reverse pedals to the rpms of the tractor so when you depress either pedal the engines rpms correspond to that movement. the models without this feature will not do that, you would have to manually use the rpms set lever to increase the engine rpms while operating the tractor.
hope this helps clear things up for you or anyone else who were interested.