As you can see here (section 4) , the 'slot' can be at any point in the section, its function primarily is to re-energise the upper surface boundary layer with 'high pressure' air to improve attachment, in this case just in front of the flaps.
basic airfoil | slat | tr | 1933 | 0454 | Flight Archive
Southampton University cover it well at page 93 of
this doc