I don't know anything much about these particular autopilots but a "preselect" in this context usually means a means of selecting a target value which the AP is supposed to fly towards and then capture and hold.
The most common "preselect" is altitude. You are at say 5000ft, you set "8000" in the altitude preselect function, and then you somehow have to make the plane go upwards (obviously
) and when it reaches 8000ft the AP will level it off. How you make it go up depends on what additional functions are available; some APs will maintain pitch only, the better ones will have a separate preselect for VS so on one of those you would set say +300fpm.
If you set say -300fpm, the AP will fly you all the way into the ground, never realising that it is getting ever further away from the 8000ft target.
Historically, a "preselect" used to be a separate little panel, with a means of setting the figure to capture. On more modern units, it is all in one box, as IMHO it should be.