There are too many details to fully explain in this blog post but this is called Downstem Duration because the note symbols with stems down are used only for duration. Up stems are the normal timing indication. These can be left out when a default up stem note is shown like the eighth note at the start. All notes are then eighths unless another note symbol changes it.
Gray overbars indicate chords in ColorTab and the low note is the default for the chord, but the timing and duration of each note can be different. A single up stem quarter note on the low string means both notes are quarter notes. A quarter note down stem extends the duration but keeps the default eighth timing. An up stem eighth on the upper chord note resets the duration.