Arduino Mega 2560 Supply Power Pins

Dear friends,
I have a Arduino Mega 2560 Board (but not genunie). I am using 5 V pin on the board to supply sensors. I am supplying 2 sensor through the same 5V supply. Today I ahve seen that there is another 2 5V on the corner of the board. Are these pins are power supply? And there is another Ground on the other corner. Can I use there ports for supply and ground?
I could not find necessarry information on this. You can find photo in the attachments

All 5V and GNDs are connected together. You have up to 800mA of 5V current available for the uC and any peripherals attached, and probably less depending on the voltage at the barrel jack due to the regulator overheating. 500mA if powering from USB.

CrossRoads: All 5V and GNDs are connected together. You have up to 800mA of 5V current available for the uC and any peripherals attached, and probably less depending on the voltage at the barrel jack due to the regulator overheating. 500mA if powering from USB.

Can Pins maked with 5V at the corner be used as 5V voltage seperately? This means there is three 5V Supply pin right? I also do not know what uC means