Hi Mario,
I guess "music theory" is used to some extent to state the obvious.
I read and played all the examples of
https://en.wikipedia.org/wiki/Passing_chord. All of which I am familiar with how they sound.
"Passing chords may be consonant or dissonant and may include flat fifth substitution, scalewise substitution, dominant minor substitution, approach chords, and bass-line-directed substitution"
Well, all of that is a mouth full and could be something to consider as a composer. Less so as a player unless you are sight-reading the music or have listened to something enough to play it note for note.
They also go on to say A diatonic passing chord may be inserted into a pre-existing progression that moves by a major or minor third in order to create more movement.
So...does it have to move by a major or minor third?
Sometimes I find Wikipedia lacking in their explanations.
Billy