The existence of a link between two class parts indicates that these two parts cannot have a course at the same time: they have (or might have) students in common.
As a precaution, EDT places a link between two parts from different partitions. Take for example a class with two partitions FL2 and LATIN. Potentially a Hispanist can also be a Latinist; EDT cannot take the risk of placing the Spanish and Latin courses at the same time. For this, EDT links by default each part of FL2 with the part of LATIN.
If you know that two class parts will never have a student in common (for example, because no Hispanist is a Latinist), manually remove the link between these two parts: eventually EDT can place the corresponding courses at the same time, which allows the obtention of better timetables.
From the display Timetable, double-click on the concerned course to enable the diagnosis mode.
Move the green matrix to the course that is in conflict: the link icon appears on the course form, in front of the concerned part.
Right-click on the link and choose Delete the link.
Note: if the parts have at least one student in common, the link is no longer optional and cannot be removed. If you want to ignore this timetabling incompatibility for some students, you can add them to the wanted course as detached students (Customized timetable).
Also see