So if it's Poly it Plays all elements?
And Mono just 1 element?
No, they have nothing to do with elements, just number of concurrent notes.
Polyphonic means multiple notes can be played at the same time on that channel.
Monophonic means only one note at a time can be played on that channel, regardless of how many elements are layered together to create the voice.
On a channel that's set to monophonic mode, if you play a new note while another note is already playing, the old note will usually be cut off so the new note can be played.
But in some cases I think the new note might simply be ignored and the old note will keep playing.
On a synth, there can be different rules about how you want to handle multiple notes-- always play the highest note, always play the lowest note, or always play the last-pressed note.
So if you're using highest-note priority and are pressing the C4 key, then press the G3 key while still holding down the C4 key, the G3 would be ignored because the highest-pressed note (C4) will be given priority.
But if you're using lowest-note priority and press the same keys in the same sequence, the C4 will cut off as soon as the G3 is pressed.
I think I saw a SysEx message in the XG documentation that lets you set which priority you want to use, although I'd have to go check to be sure, but the default is probably last-note priority.
The specific behavior about how the new note is sounded will depend on the legato and portamento settings, but this could also vary by model.
If legato mode is in use, the new note will sound without retriggering the amplitude envelope-- that is, the ADSR or Attack-Decay-Sustain-Release which shapes the amplitude of each note. [Edit: I'm specifically referring to playing a new note while an old note is still being held down. If the old note is released before the new note is pressed, the amplitude envelope will be triggered normally.]
If portamento is turned on, the sound will slide from the old note to the new note. [Edit: Again, this assumes that the second note is played while the first note is still held down.]