Quick answer: In some cases, different scales are appropriate to be used to derive chord substitutions at a certain place in your arrangement. These different scales will produce different sets of substitutions. The prime symbol indicates the use of a different scale which has the same base chord as another entry in the substitution drop-down. The base chords of those sets will sound the same, though the tensions will be different.
Differences between Bb and Bb' chordsAfter Liquid Notes performs a harmonic analysis of your piece, it generates a set of chord substitutions which you can use to change the sound of your music.
The available substitutions depend on the underlying musical scale from which they are derived. Sometimes, different scales fit to your music, and they can be used to build the same base chord (Bb for instance). But when you start to add tensions to the chord, the different scales will add up to different tensions.
For example, consider the following example:
In the picture above, the substitution drop-down contains a Bb and a Bb' symbol. In this case, when you select the Bb symbol, the current scale in Liquid Notes changes to Bb Major (as indicated at the bottom right of the application window in the current scale indicator):
The available tensions for this Bb chord are thus derived from the Bb Major scale: sus2, sus4, 6, maj7, and so on (visible in the Bb sub-menu in the picture above).
But when you select a substitution from the Bb' menu, in this case the current scale changes to Eb Major:
In this scale, there is also a Bb chord, starting from the 5th scale degree. It's tensions look different now: 7, add9, 7 with 9, ... They are that from a dominant chord.
In other words, the prime symbol indicates the use of a different scale which has the same base chord as another entry in the substitution drop-down. The base chords of those sets will sound the same, though the tensions will be different.