[atzebmw]

OK, I will download PSdZData Full and extract to PSdZ\data_swi as you recommended

What is the purpose of PSdZData Lite - is this required for diagnostics?

If your ISTA is already working and you downloaded already the psdzdata Lite you now only need E-Sys and a Launcher for coding your trailer coupling retrofit.
You don't need the psdzdata Full.
In summary:

E-Sys + psdzdata Lite= Coding trailer coupling
E-Sys + Launcher + psdzdata Lite= Coding some other things
E-Sys + Launcher + psdzdata Full= Coding + flashing ecus
ISTA without psdzdata or SDP or BLP=Diagnostics