guitar mode is referencing each string
EADGBE in order from highest frequency to lowest frequency. So the high
E (thinnist string) would be
1E, then the
B string would be
2B, then the
G string would be
3G, ect all the way to the low
chromatic mode just tells you what note you are playing and how close it is to that note. i.e.
A, A♯/B♭, B, C, C♯/D♭, D, D♯/E♭, E, F, F♯/G♭, G, and G♯/A♭
If you are tuning your guitar to the standard
EADGBE, you would use
guitar mode. However if you wanted to tune your guitar to drop D
DADGBE, you would use
chromatic mode because the low
D is not a standard tuning for the guitar.
Another use for
chromatic mode is to tell what notes you are playing on your guitar. It is very useful when you start learning guitar and you don't know what note you're playing. Also if you feel like notes are going out of tune higher on your guitar you can check with the
chromatic mode. If your string is in tune and your higher notes are out of tune you may have an intonation problem or you may have to change your strings.
guitar mode is for tuning the strings on your guitar to standard tuning. While
chromatic mode shows you the current note you are playing and how out of tune you are with that note so it can be use for more than just tuning your guitar (although it still can be used that way).
My advice would be is use
guitar mode until you have a good grasp of what each string should be tuned to then switch to
chromatic mode because it can do a lot more.