Hello Gatto,
"Preselect image pairs" option is available in the Align Photos dialog and is fully automatic. The idea behind this option is that the photos are first matched using a lower accuracy setting, which is enough to detect whether a given pair of photos overlaps or not. After the set of overlapping photos is identified, they are finally matched using a higher accuracy setting.
This usually helps to speed up photo alignment step, especially for mosaic-like capturing scenarios.
With best regards,
Dmitry Semyonov
AgiSoft LLC