Excerpted from The Alex de Grassi Fingerstyle Guitar Method
In third position or higher, it becomes possible to combine open and fretted strings to play ascending/descending intervals on nonsequential strings. This technique is called cross-string picking. For comparison, first play the third-position G-minor scale in Example 3, which uses all fretted notes. Then, play the third-position cross-string G-minor scale in Example 4. The first ten notes of the scale alternate strings back and forth from lower to higher to lower again.
Once the Bb is played on the third string, however, the remainder of the notes in the scale progress sequentially from lower to higher strings. This is because the pitches of the top two open strings, B and E, are not part of the G-minor scale, so it’s not possible to utilize the cross-string technique in this part of the scale. To match the more legato texture of cross-string playing, these notes are connected to each other by hammer-ons and pull-offs.