key center / modal center

the "key center" relates to the start of the piece where you get a "key signature"
"the modal center" is where you place accidentals in your piece to change the mode of the song .
EX:1
If a song is in C major , thats the key center ,
Now you can add a "B flat" if you like , but by doing this you change the "modal center "to C mixolydian , "The key center" remains C major.

EX:2
Now let's start with an F major "key signature" Here the "Key Center " is F major .
you could unflatten to "B flat" to "B natural " but once again we are changing the "modal center" this time To "F lydian "
In this instance our root note remains F just as in the Previous case our root note remained "C"