For example, if a passage of music is marked "sub. forte," it means that the music should suddenly become loud. Similarly, if a passage is marked "sub. piano," it means that the music should suddenly become soft.
Sub. can also be used to indicate a sudden change in tempo. For example, if a passage of music is marked "sub. allegro," it means that the music should suddenly become faster. Similarly, if a passage is marked "sub. adagio," it means that the music should suddenly become slower.
Finally, sub. can also be used to indicate a sudden change in articulation. For example, if a passage of music is marked "sub. staccato," it means that the notes should suddenly be played in a detached manner. Similarly, if a passage is marked "sub. legato," it means that the notes should suddenly be played in a connected manner.
Sub. is a versatile musical term that can be used to indicate a variety of sudden changes in musical performance. It is an important term to be familiar with if you want to be able to accurately interpret and perform music from a sheet.