After entering or changing a chord shape in the fretboard, the tone on the lowest string is considered as the root.

With the button inversions and slash chords you can navigate through the list of the different chord interpretations. The corresponding chord symbol shows up above the fretboard and the spots of the chord shape will display the according notes or intervals.

If you choose to interpret a chord as a slash chord, the lowest note is added to the chord symbol after a slash. In return the interval it represents will not show up in the chord symbol anymore*. Example: Cmaj7(9)/ D has the 9th in the bass and therefore can be written as Cmaj7/ D .

*) If you prefer to write the full length chord symbol in front of the slash on your lead sheet, feel free to do so.

Which interpretation is the right one?

In most cases it's simply the shortest chord symbol, which is also the easiest one to read. But you should ask yourself what tone a bass player would play to it (even when there is no bass player playing along). Consider that tone as the chords root.

If the bass tone is the same as the lowest tone of the chord shape and the chord symbol seems too complex or doesn't make sense theoretically, you may consider to choose the easiest possible interpretation as slash chord.