problems about osc in ep3c120

I am working with EP3C120F780C7N FPGA development board.I need your help,I

am confused with this issue:

which document decribe the pins of MAX II,especially "clkin_50_en" and

"max2_clk"?

which state clkin_50_en is on when the board is powered?

how can I control the signal "clkin_50_en"?

which clock is MAX II's gclk,clkin_24 or max2_clk?

best wishes!