The ZOH TF above is a link between continuous and discrete domains in hybrid systems. This is the most convenient mechanism for representing a hybrid system in transfer function form. There is not, of course, a one-to-one relationship between s\$s\$ and z\$z\$ domains, hence it's a mathematical convenience. In the relationship above, the exponential term should be negative (not positive as given), giving a z\$z\$-equivalent of 1-exp(-sT) as (z\$1-\exp(-sT)\$ as -1)/z\$(z -1)/z\$ to be included with purely discrete blocks (filters, etc)and and the 1/s\$1/s\$ part of the ZOH should be included with the other continuous s\$s\$-blocks. The s\$s\$-TF of the continuous elements is then transformed into the z\$z\$-domain, giving an overall z\$z\$-TF.
