The ETT90 is a Nordic test program to increse haul weight for long timber transports. To increase loading weight the crane is removed on the truck and the bunks cant be moved.
The timber trailer feature hydraulic moving bunks that moves so you can reach them with the timbertrucks crane without a problem. Just move the backbunk forward, load it and strap the timber in place and move it backwards to load the other bunks.

Required truck for this pack: NMC Griffin https://www.farming-simulator.com/mod.php?lang=en&country=us&mod_id=152351&title=fs2019

Price ETT90: 60 900$
Price Timber Trailer: 45 000$
Different Wheel setups