For the units:
Engineers are classified as "Mobility, counter-mobility, and survivability". This means they get in your way, get things out of your way, and/or put things in front of you to stop bullets.
Mechanical I'm not sure about. If you mean mechanized, this typically means heavier tracked vehicles ('motorized' means wheels; 'light' is walking). Note that an engineer unit may be mechanized.
If you mean a maintenance unit, they mostly fix vehicles (although there are other possibilities).
As a 92Y, this should only affect types of supplies used, and the field conditions you will be exposed to. The job itself should not vary all that much.
If you graduate after they deploy, you probably will find yourself in the rear detachment. That means you would be training, supporting the unit, and/or maintaining the facility and equipment they didn't take with them.
This is a ten level task